Chimera Damage is doing its pure damage (125% weapon damage) and on top of it Instantly deals 40% of the damage done by your Serpent Sting. The T9 2 piece bonus set is adding damage done by your Serpent Sting ability can now be critical strikes. I see in the recount log that the portion of "chimera Shot - Serpent" is showing crit, but the percentage of the crit is lower than the pure Serpent sting crit.
I wonder if indeed the T9 2 piece bonus is adding the critical strike to the Chimera's 40% of the damage done by your Serpent Sting, or there is something on top of that?

I have noticed few take IHM as well., as I raid with 2 other hunters, sometimes 3. I know having 3-4 hunters in a raid is not exactly optimal. Lol. I took it upon myself to take and glyph IHM, to help better my other hunters I raid with, but my average dps did not change while taking IMH, the only time I notice a change is during execute phase, when I am getting fewer kill shots in due to my glyph. If you raid with 2 other hunters, I'd recommend keeping it, as it certainly isn't hurting you as long as you only pull the points from ImpSteady, but I wouldn't recommend taking IHM unless you glyph it as well.

The benefit of talenting/glyphing Hunter's Mark diminishes greatly when you need to target switch to fairly low-hp mobs. The following fights tend to have phases where you swap targets and it's not worth casting HM at all, so you end up losing dps:

Lord Marrowgar - Bone Spikes
Lady Deathwhisper - All Fanatics and Adherents
Gunship - Everything
Deathbringer Saurfang - Blood Beasts
Professor Putricide - Ooze and Gas adds
Valithria Dreamwalker - Everything
Sindragosa - Ice Tombs
Lich King - Ghouls, Ice Spheres, *Raging Spirits

Considering it's a wash for personal damage anyway, it's easier to opt out of a "raid dps boost, sometimes" in favor of a "personal dps increase all the time."

This is true, however you also want to take in to consideration about which part of the fight is the most crucial.

For example, on Marrowgar, getting those bone spikes down as fast as possible (during his bone storm) is the key to winning the fight as a dps. In this situation, ISS > IHM even with multiple hunters.
For Lady Deathwhisper, - this fight depends more on your role during phase 2. However, if your guild is struggling with phase 1, it's a different issue. But in phase 2, if hunters job is to totally focus the boss the entire time, then IHM > ISS, but if you and/or your other hunters turn off the boss to dps adds, then ISS > IHM.
Gunship is a non-issue.
Deathbringer Saurfang is another situation where ISS > IHM. The crucial moments of this fight are taking down blood beasts as fast as possible so that your raid gets more dps time on Saurfang.
Professor Putricide - This is an interesting case because a lot of people will say that phase 3 is the most crucial phase where IHM would be the clear choice, but phase 3 begins with both an ooze and a gas and you don't put up HM on them. So, the "crucial dps time" of this fight starts when those two oozes come down and ends when Professor Putricide dies. In this case, I would suggest ISS over IHM as it can lead to a little faster start. Not to mention the first transition between phase 1 and 2 is also considered a crucial dps time and a time when IHM is worthless.
Valithra - dps is not really a big issue here, but yes IHM is worthless
Sindragosa - it's true that you don't HM the ice tombs, but for this boss, I would venture to say that IHM > ISS and here is why. The last phase of Sindragosa is the key to the fight especially on hard mode. Hunters really shine here because there is a lot of time spent sub-20% where the kill shot glyph is really going to give you a lot of extra dps. Also, I'm pretty sure most guilds keep there hunters exclusively on Sindragosa and ignore ice tombs completely, so in this case, IHM would actually be better than ISS (although it's too bad kill shot isn't affected by ISS).
Lich King - there really isn't a single crucial dps time for this fight. From 100% to 70%... yes IHM is going to be better, but during the transition phases with the raging spirits, ISS is going to be better, and most people would agree that getting those Raging Spirits down faster is more important than pushing him to 70% faster. Also the above poster forgot to mention the Valkyr which is a crunch dps time where you don't cast HM either so overall I would say ISS > IHM for LK normal or hard.

So I agree with Mako that points in ISS instead of IHM will more *overall* damage on the fights mentioned, BUT I believe IHM is still more valuable for certain fights because it gives your hunters the better damage where you need it the most.
TL;DR version - with 2+ hunters in the raid, IHM is better than ISS for Sindragosa and (situationally) Lady Deathwhisper because the crucial parts of these encounters may not involve you or other hunters hitting anything but the boss and it's the most important part of the fight.

They should definitely be marked if there are MM hunters DPSing them, but not IMO by the hunter with IHM. Leave his mark on the boss, so only one GCD is used to mark when an add spawns instead of 2 (one on the add and then one on the boss again because the IHM hunter marking the add would remove his mark from the boss).

Ooze variable can throw a wrench in this system though since the hunters can end up split between 2 targets.

I think people simply undervalue IHM because in the Best DPS threads and BiS gear setups another hunter is assumed to be providing the buff. And people tend to copy the specs they see in these threads without analyzing every talent point.

People have listed all sorts of fights where you target switch. Well, you could just the same list all the fights where you move and in those fights you steady shot less. In fact, the fights where you target-switch a lot are often fights that require movement. People also tend to forget, that when a target is going to die in less than 8 seconds, you are often times better off using arcane shot as piercing shots won't have the chance to fully tick on that target.

I haven't done any of the ICC fights on heroic, so there might be differences there, but I have a hard time believing that a talent that adds more DPS per point even in a 1-hunter situation on a tank and spank fight (ideal for ISS) and only requires the presence of a debuff that is amazingly good by itself would be worse than ISS in a situation with 2 or more hunters. Hunter's mark doesn't just add 500 AP (650 talented), it also makes marks hunters and their pets do 5% more damage. It's not so ridiculous to expect that it will be up on our targets most of the time.

I'm not saying IHM is always better than ISS. What I am saying is that you shouldn't just look at some top hunter parses and copy their specs. There are many factors to consider, not the least of which is that hunters are judged by their DPS first and foremost and recount and WoL don't add 50 DPS per hunter to yours just because you spec'd for a talent that resulted in a raidwide DPS boost. I also can't count the number of times I've looked up a hunter in a top 25 guild and saw suboptimal gemming, being over the hit cap, or using talent specs that were clearly suboptimal.

This isn't to say those hunters are bad players. Often times what you do in a raid has a far bigger impact on your performance than the theorycrafting you do outside of raids.

P.S. The last time I checked, talenting IHM was worth it but glyphing it was not. This may have changed as I have not raided for a couple of months now.

So, this seems like a silly question to me, but I feel I need to ask it anyway. In raid today, one of the "I am better than you, and I know everything" hunters said to me... "Heroism doesn't really benefit MM hunters." This, to me, sounds kinda nuts since everyone's dps (hunters included) seems to shoot through the roof when it's up. Anyone who can enlighten me on this subject? I mean, if it doesn't benefit us I would love to know why.

In my post I explained why I think it is better to spec in to ISS over IHM on certain fights. Think of it this way. Let's say you are the hunter that specs in to IHM. During the time that you are damaging a target that you don't personally mark, those three talent points are completely and utterly useless.
I'll use my own profile on the spreadsheet as an example - my spreadsheet dps is 13424 with 3/3 IHM or 13411 with 3/3 ISS. As you can see, the dps is pretty much negligible. However, if I change targets, I'm basically 0/3 in both. So if I spec out of both on the spreadsheet, my dps is 13300.

The purpose of my post wasn't to emphasize that ISS does more damage, because no matter what the fight, you always do more damage to the main target (unless it's Valithra where you don't have a main target). So for overall damage, if you have another hunter with you, IHM is going to be a damage increase. However, the point of my post was to emphasize that there are certain times where damage is more important than other times.

Specifically Putricide hard mode: in my opinion, the dps you put out on the adds is more important than the dps you put out on the boss. Therefore having ISS instead of IHM is better for raid success. If you disagree with add damage being more important, stay with IHM.

And anytime you are moving, the chance for ISS proccing goes to zero making the talent completely and utterly useless...

Neither talent is completely and utterly useless. My point is that on a single target tank and spank, which is the best case scenario for both talents, IHM > ISS by about 17dps per talent point, YMMV. And on a fight that's not a tank and spank, they both suffer.

Now add a second hunter, and on a tank and spank IHM is worth an extra 150 DPS for each hunter other than you (since they don't have to sacrifice anything for the benefit), on top of it being better DPS per point for you. That's not something you can just ignore because at some point in the fight for a few seconds you were DPS'ing a target that wasn't marked.

This isn't even true. There is about 4 seconds of moving you can do every 10 seconds whenever you are firing aimed shot or chimera shot without affecting ISS at all. Also you can disengage in between steady shots and still get 4 steady shots in between your aimed/chimera combo (or if you do steadyx2, aimed, steadyx2, chimera... the same concept still applies). If you have to move during the middle of shooting steady shot, AND disengage is on cooldown (or you just can't afford to move the full length of disengage), THAT is the only time ISS suffers.
However, that isn't even close to the point of my post. Maybe I'm not making mysyelf clear about the purpose of my post. That point is, damage is sometimes more important on adds than on the boss and in those scenarios ISS is better to have so that you can kill those adds faster. Please re-read my bolded phrase.
Prime example being Saurfang Blood Beasts. You don't really have to move much, but as soon as those beasts spawn, you target your specified target and dps it down. If you happened to get an ISS proc from using steady shot on Saurfang before they spawn, you just gained a nice 15% damage bonus to the aimed shot you dealt to the blood beast.
To reiterate - OVERALL DAMAGE is not what I'm talking about (please re-read my bolded phrase). Obviously if you have IHM, you'll probably do more overall damage, and any hunters in the same raid with you will definitely do more overall damage.

Another example, but in reverse would be the LK fight in phase 1. Sure, you can multi shot the drudge ghouls to up your Recount damage. Heck, you could even volley during phase 1 and have higher Recount damage. But does that help the encounter? Certainly not. The whole idea in phase 1 is to push the LK to 70% as fast as possible. So you lose out on Recount damage in order to do what's more important for the raid.

I hope that clears up any misconceptions about what I'm trying to discuss.

P.S. *Note - this is off topic from the rest of my post* Using the hunter spreadsheet, my current gear puts me at 13424 dps, spec'd into IHM and 13411 dps spec'd into ISS instead of IHM. 13 dps loss for ISS on a single target. Assuming a hunter had the exact same gear as myself, he would do 13528 dps if he was 3/3 ISS and I was 3/3 IHM, which means IHM is giving him a 124 dps upgrade. So the above poster's generalizations aren't too far off exact numbers. However, I would surmise that most hunters don't have H 25-man gear and in such a case, the dps gain might be less than the numbers I've shown here.

- numbers above are pulled straight from Shandara's spreadsheet, using my current gear, and modifying raid buffs to show HM is not improved or glyphed from another source.

He got 9 Exploit Weakness and you got 2 (you probably read "Buffs cast" instead of "Buffs gained". But since your pet also gets the buff, it counts double.). This alone could explain the 1k dps difference, it's just luck. You also got 1 Vile Gas more than he did. Don't sweat it.

On the topic of IHM vs. ISS:

Saurfang, and maybe Marrowgar and Deathwhisper are really pretty much the only fights where I would agree on ISS > IHM. And on Marrowgar it's really only important on the whirlwind phase while on Deathwhisper it depends on your role in phase 2. On Putricide IHM would still win since you can (and should) mark oozes there. Dreamwalker and Gunship are irrelevant because dps isn't a matter. But for the record, ISS would be better on Dreamwalker since bursting skeletons is the most important thing for dps on this fight and IHM would be better for Gunship because the faster the mage dies, the faster the fight is over (and you can mark him while flying). Sindragosa IHM would still be better in my opinion because the damage in phase 3 is really important and hunters should dps her during that phase all the time.

So that's like 3/11 (don't know LK H, so no judging there) where ISS wins. I think you should just spec according to the fight you are wiping on^^