Measuring the speed via number of breaths is misleading because they are quite random. You have 20 secs SI, 12 sec barkskin. Are you killing Shadron within lets say 40 seconds after Torment comes in? If yes then you dont need FR (i guess, not sure if Barkskin alone is enough with MotW and FR aura). If not, then you rely on external cooldowns. Or you rely on luck.

We are not that fast, on our attempts i usually take 1-3 breaths without any cooldown available, so i use FR set (345 fr, 39k hp during fight).

I think you are blending the "common wisdom" for 10- and 25-man a little bit. I've not used my full FR on 25-man, because (as a non-LW) it represents a significant loss of mitigation and avoidance over my normal gear, which (as I discovered over the past month) really does add up to a lot of incoming damage. I can't make the 25-man "no external cooldown" 58k health/340 FR anyway. (Remember the effective health needed is substantially bigger on 25). I just use the helm and cloak enchants, aura, and and a magic resistance flask (Chromatic Wonder for 25-mans).

In 25-man, I call for Pain Suppression on the first breath, since it offers substantial damage reduction rather than merely more health to heal; and Hand of Sacrifice on the second. I then use SI and Frenzied Regeneration for the third and fourth breaths, and barksin and a fire pot are reserved for a fifth if we get there, or for an emergency where I'm not fully topped off. Generally if things are going alright we will kill Shadron by the third breath, so the rest after that is just to help the healers catch up on healing.

I use an entirely different gear set and cooldown rotation on 10-man because the dynamics are so different and (for us anyways) the drakes are up much longer. I use exactly 340 FR (cape and helm enchants; level 70 FR chest and pants; Flask of Lesser Resistance; totem or pally aura) and roughly 49K HP buffed, or 51k-ish when I have Commanding Shout or Blood Pact. For me, 10-man is much more about offsetting my incoming damage while Vesperon and his Acolytes are alive, rather than merely surviving breaths.

In 10-man, I use barkskin on the first breath, fire pot on the second, and SI on the third and fourth. Using Barkskin first means you will usually have it again by the 6th breath since the cooldown is short. I use a healthstone immediately after any breath that is not mostly resisted. Frenzied Regen is usually paired with SI, but not if I get two good resists. I only ask for a cooldown from my healer (usually a holy pally or disc priest) when multiple Vesperon acolytes are up, around the 5th breath; that way, the damage reduction applies to my physical damage taken as well when that starts to be a big concern. (That's also when I use the dodge effect on my Monarch Crab, to make it easier for my healer to catch up). Then barkskin is back up again, and perhaps a nightmare seed sometime after that if the drakes are really dragging. By the time I am out of emergency buttons Vesperon is typically dead and the DK tank takes Sarth off me.

It was honestly more of a concern on the raid leaders part; as my gear isn't quite where the old MT's was.

For both 10 and 25man I am NOT using FR, I am using a Polar set (puts me to ~51k hp raid buffed on 10 man, and about 36% dodge w.o procs).

I usually get about 350k or so threat by the time the 2nd drake is down, so I don't see a concern with it, but appearently the raid leader did.

Is there any way that I can really assure that there wouldn't be a problem? I don't see any issues with how much threat I was pulling in my polar set, they had simply stated "we don't want to take any risks" (we wiped for a good hour as we had 3 totally new tanks to the fight).

I'm still not quite sure why you don't want the drake tank to take Sarth after the drakes are dead; for me, the opportunity to help on adds, DPS, BR, IV, etc.--and the break from the monotony of calling lava waves, moving left, moving back--is preferable. Anyway, if you have a 350,000 threat lead when the DPS starts DPSing, and (I'm making this number up) Sartharion lives for two minutes after the DPS is on the boss, that means your DPS would have to be doing nearly three thousand more threat per second than you for them to catch up by the end of the fight. I'd say that's impossible.

Edit: Here's a formula you can give your raid leader. They catch up to you when
DPS TPS = MT TPS + ((MT threat lead when DPS begins)/(length of fight after DPS begins))

In all our 3D kills (10 and 25man) I've tanked Sarth start to finish. After the healing intensity during TTorment, the rest of the fight seems trivial to heal and I've never felt remotely threatened. It probably helps that every time we've gotten that far we've had no one dead and healer mana has been fine.

The only time I've ever had an issue with threat is people being dumb and starting dps before Sarth even gets to me. I'll usually start using a high threat weapon/idol and pop Enrage/Berserk while positioning. After the first wave comes everyone switches to drakes and I put tank weapon/idol back on.

Using FR is the same reason it is optimal to use high armor on a hard hitting boss. Reducing the size of the big hits makes healing easier than simply living through them. For 10 man it makes tanking Sarth trivial, for 25 man it makes our cooldowns viable to live through a max breath.

There's no sense in being overly proud to tank Sarth from start to finish. Just because something can be done doesn't mean it is optimal to do so. The drake tank will pretty much always have better stats for tanking physical damage (again damage mitigation rather than just survival). Brez and Innervate alone make having them taunt worthwhile. Tanking blazes during enrage in FR is a bonus.

I can somewhat emphasize with this, but at that point the damage received is trivial enough that even a 10man geared tank can tank it, so don't think too much about it.

Again, this really depends on whether we are talking about 10- or 25-man, and your group composition. In 25-man you will generally have a lot of options for external cooldowns and plenty of HPS going onto the main tank. In 10-man there is more strain on the healers; furthermore, some group compositions don't have any external cooldowns available (say, resto druid/resto shaman healer combo), and some group compositions with lower or fewer DPS might have Shadron and particularly Vesperon up for awhile, in which case the FR becomes more valuable.

Note also that the polar gear is included in most of the stamina/FR sets people have talked about in this thread. Amusingly enough, I actually have more frost resistance than fire resistance in my fire resist set. Edit: how is the polar gear "cheaper"? The enchants are quite cheap compared to epic craftables, and the flasks are made from cheap mats.

It's the fact that you have to either A) swap the enchants out for that one encounter or B) get the 2nd set of gear and spend the gold to enchant/gem it. It doesn't seem worth the effort to get the FR set if you can easily do without it.

This has always been the main point. With an FR set over 340 you really don't need any outside cooldowns in either 10 or 25 man versions.

For 25 man, I usually only take 1-3 fully buffed breaths and only one cooldown is needed to survive even a max damage breath. I'll use SI on the first breath and watch the timer on it, the buff is usually still up for the second. On the third breath I'll use Barkskin. Just to be safe I'll usually tell a priest to be ready with GS just in case a 4th is needed, but I very rarely end up calling for it.

In the 10 man, you really only need the cooldowns to survive a max breath. Most of the breaths won't put you in danger even fully buffed. I'll still use them exactly the same as 25 man just to be safe. However, as already mentioned, using Barkskin first will allow it to come back up again for another.

Removing reliance on external cooldowns is a huge gain in stability for the encounter. I'd much rather not have to rely on someone using their cooldowns too slow or hoping for RNG to be kind and not deliver max damage breaths.

Making the FR set really isn't hard. I still use the 70 epic gloves and have a green ring and bracer (with LW enchant). I put the FR enchant on an old tank cloak and the FR enchant on my [Hood of the Exodus]. This is the hardest fight in the game and nothing else needs the full tank enchant - it's only a few extra points anyway. Add on a resist flask and you're there.

The danger in 10-man is not just a big breath. It's taking a big breath and having the healer be unable to heal you up before he does a few swings. If a shadow fissures spawns beneath him, he may have to move, etc.
So, in 25-man you use cooldowns in order to not get 1-shotted. In 10-man you use them so you don't dangerously low on health.