I've done a few math about the new 4pc PvE set bonus:
Since it the 0.2 seconds will be taken from base cast time, we have:

Haste Effects	Total Haste	Base Cast	4pc Cast
No Haste	1	2	1.8000
ISS + Raid Buff	1.265	1.5810	1.4229
BL + ISS + Raid Buff	1.6445	1.2162	1.0946
RF + ISS + Raid Buff	1.771	1.1293	1.0164
Glyphed RF + ISS + Raid Buff	1.8985	1.0535	0.9481

In order to free 3 GCDs per CS cycle, or 1.25s cast SS, one would need 13.84% haste from gear and/or Pathing. It's also notable that with the bonus, Rapid Fire glyph becomes pretty useless, since RF would bring SS cast to 1.01s.

Edit: clarification and removed wrong information.

The value of Mastery definitely increases in 4.0.6. Blizzard buffed mastery's impact on WQ by 17% from 1.8 WQ per point in mastery (179.28 mastery rating) to 2.1. Thus, each point of mastery is about 17% better. The change buffs my WQ from 24.85% to 28.99%.

Without factoring in the T11 4-set bonus, the increase to mastery should not really change our priorities much yet. They are still the same. After making sure to get hit capped and enough haste to have a 1.5s static SS cast, you will still prefer crit over mastery. One thing to keep an eye out is how close folks in heroic T11 gear may be getting to the CA crit cap. But once the CA crit soft cap is reached, the value of additional crit will be a little diminished.

With factoring in the T11 4-set bonus, our base SS cast is 1.8s. This has several possible choices listed below. This analysis used a quickly thrown together T11 heroic set with the T11 4-set bonus. With no reforging, the stats of the set are:

DPS: 27090 KS glyph, 27312 CS glyph
Hit: 100%
Crit: 36.36%
WQ: 33.48%
Haste: 8% from gear plus 2% from Pathing
Static fps: 4.41
SS cast: 1.292
SS BL: 0.994
SS RF: 0.923s

With no reforging, mastery is the lowest secondary stat, but it was close behind crit and haste:

Haste Rating + 1	1.327
Crit Rating + 1	1.240
Mastery Rating + 1	1.139

1) Keep our rotation as is with a static 1.5s SS cast (or a little below - by static I include ISS and raid buff). This requires no haste from gear and Pathing to make it possible. With no haste from those sources, our SS cast time is 1.423s. This really is not the advisable option since we are going to have haste on gear, and this option really devalues the remaining haste after reforging as much to crit and mastery. Furthermore, although we will have the same rotation as before, with less haste, our focus regen will be lower to support the rotation and we will have less autoshots with less WQ procs.

With the gear set above to dump haste, I moved 2 points in Pathing to Term and reforged the haste to crit or mastery. The only other reforge was the one piece I could reforge from mastery to crit, which was a small gain. The result is:

DPS: 27088 (-2) (didn't do CS since intermediate case)
Hit: 100%
Crit: 37.47% (+0.45%)
WQ: 35.4% (1.92%)
Haste: 5.16% from gear plus 0% from Pathing (-5.0% total)
Static fps: 4.2 (-0.21)
SS cast: 1.353 (+0.061)
SS BL: 1.041s (+0.047)
SS RF: 0.966s (+0.043)

This haste/focus regen reduction lost 1 AS, 2 SSs and 7 autoshots, but did gain 4 WQs from the additional mastery. It left us with more haste than we needed for the 1.5s static SS cast, but not enough still for a 1s SS cast during RF, although it is close.

With hand changing the remaining haste into crit up to the CA soft cap and then mastery to simulate using non-haste gear, the results are:

DPS: 27197 (+107) KS glyph, 27375 (+63) CS glyph
Hit: 100%
Crit: 40.6% (+4.24%)
WQ: 35.74% (+2.26%)
Haste: 0.58% from gear plus 0% from Pathing (-9.6% total)
Static fps: 4.02 (-0.39)
SS cast: 1.415 (+0.123)
SS BL: 1.088s (+0.094)
SS RF: 1.010s (+0.078)

This further haste/focus regen reduction lost about 1 AI, 2 AS, 4 SSs and 13 autoshots, but did gain 2 WQs from the additional mastery for both glyph cases. The much higher crit rate though made up for the loss in shots. The other nice aspect here is that at 1.415 SS cast time, when a 5th SS shot is needed in a CS cycle to regen focus, it almost fits in that shot without pushing back the CS. In this case, the CS is only pushed back by only 0.075s under ideal conditions.

2) Add haste to get to the 1.25s static SS cast. This would require 13.84% haste total from gear and Pathing. This is 12.71% from gear with 1/3 Pathing, 11.60% with 2/3 Pathing, and 10.52% with 3/3 Pathing. Doing this has the benefits of saving another GCD per cycle. With the additional special per cycle and increased autoshot frequency, there will be more WQ procs increasing the value of mastery more. This would also result in your SS cast being below the GCD during both RFs and Bloodlust. This does waste a little of the haste from those dynamic effects though it does ensure an additional GCD to be saved during dynamic haste conditions. To accomplish this, I had to reforge enough crit and mastery to gain 3.6% haste. The result is:

DPS: 26975 (-115) KS glyph, 27206 (-106) CS glyph
Hit: 100%
Crit: 34.9% (-1.46%)
WQ: 30.61% (-2.87%)
Haste: 11.96% from gear plus 2% from Pathing (+3.6% total)
Static fps: 4.57 (+0.16)
SS cast: 1.246 (-0.046)
SS BL: 0.958s (-0.036)
SS RF: 0.890s (-0.033)

This haste/focus regen increase gained 3 AS, 1 SSs and 6 autoshots, but did lose 9 WQs from the reduction in mastery. That and the lower crit rate was a huge factor in DPS loss. As can be seen, a lot of the value of the dynamic haste is lost since the SS cast time drops well below 1s during dynamic cast times. Hence, I do not think that currently trying for the 1.25s SS cast time is a good option since we have to currently give up too much crit and mastery to achieve it. Maybe later with more stats on higher ilevel gear it would be a better option since you would get the haste "naturally", but for now it is not a good option. This is the extreme case from case 1).

3) Tune your haste so that you always have better than the GCD cast time on SS during dynamic haste effects. As it worked out, the default case listed above is pretty close to this situation. You only need 9.46% haste from gear and Pathing to achieve this. Thus, with 2/2 Pathing, the default case just needs to lose about 0.7% haste from gear (or 90 haste rating). With reforging one item with 75 haste rating to crit, the result is:

DPS: 27174 (+84) KS glyph, 27400 (+88) CS glyph
Hit: 100%
Crit: 36.78% (+0.42%)
WQ: 33.48% (0%)
Haste: 7.41% from gear plus 0% from Pathing (-0.59% total)
Static fps: 4.38 (-0.03)
SS cast: 1.299 (+0.007)
SS BL: 0.999s (+0.005)
SS RF: 0.928s (+0.005)

This small haste/focus regen reduction lost 1 autoshot, but did more damage due to the extra crit for the KS glyph case. For the CS glyph case, the timing is such 1 AS replaces a SS.

4) Tune your haste so that you have a 1s SS cast during RF so that none of the benefits of the dynamic haste is lost. This requires 1.64% haste from gear. This is similar to case 1 except that you have a little haste from gear. The results are:

DPS: 27254 (+164) KS glyph, 27289 (-23) CS glyph
Hit: 100%
Crit: 40.55% (+4.19%)
WQ: 34.07% (+0.59%)
Haste: 1.75% from gear plus 0% from Pathing (-8.9% total)
Static fps: 4.07 (-0.34)
SS cast: 1.398 (+0.106)
SS BL: 1.076s (+0.082)
SS RF: 0.999s (+0.076)

This haste/focus regen reduction lost 1 AI, 1 AS, 3 SSs, 11 autoshots, and 3 WQs due to the small increase in mastery with the loss in shots. The much higher crit rate though made up for the loss in shots though. The other nice aspect here is that at 1.398 SS cast time, when a 5th SS shot is needed in a CS cycle to regen focus, it fits in that shot without pushing back the CS. Thus, there is no CS delay when casting a 5th SS in a cycle.

5) Tune the haste so that when you need to cast a 5th SS in the CS cycle, it can be cast without delaying your CS under ideal conditions. As already stated, the requirement for this case is very close to that for a 1s SS during RF, case 4).

Conclusion:

To start, the heroic T11 gear favors the CS glyph a little over the KS glyph.

How does the 0.2s reduction in SS base cast time impact an MM hunter. To be honest, it really does not change much for us at all beyond requiring less haste on gear. With all the scenarios I looked at with heroic T11 gear and constant glyph, the max change was 164 DPS or about 0.6%. It does not really change our rotation since its not worth getting to a static 1.25s SS cast time since we have to give up too much crit and mastery to achieve it and since it reduces the value of dynamic haste effects. It is still true that our haste floor is a 1.5s SS cast (which can be exceeded with 0% haste from gear and Pathing with the 4-set bonus) and that the haste soft ceiling is that that to get to about a 1s SS cast during RF. The main difference is that the optimal haste region has been shrunk from a 7.5% spread to about a 1.75% haste spread.

For the KS glyph case, it appears that it is optimal to operate at about 1.75% (or a little more) haste from gear and Pathing so that you are still doing your current 1.5s SS cast rotation but with some dead time in it for inefficiencies. It also provides plenty of room to switch out an AS for an SS, on a cycle where you need to regen focus, without delaying your SS cast. Finally it results in a 1s SS cast during RFs.

For the CS glyph case, it seems optimal to have enough haste for a 1s SS cast during BL (about 7.41% from gear and Pathing).

Note that this analysis is not complete since it does not include the impacts of possibly casting non-MMM proc AIs in the rotation.

Source: MMO-Champion - Patch 4.0.6 - PTR Build 13529

Looks like they finally fixed the MM proc for Aimed Shot and both Chimera and Aimed got huge love now that they scale off of 70ish% RAP.

My question is that would be it theoretically be possible AND viable to skip Arcane Shot now and get enough haste to pull a wotlk like "rotation" where it's Chimera, aimed (hardcasting if need be) and steadies and only use arcane when moving, given how big of a damage boost Aimed got, or would the loss of reseting auto-shots deter us from it?

One major point about those changes is that MM damage will become very spiky, based around CS and AiS. I could have sworn I read that MMM was considered one of the "knobs" GC referred to for adjusting specs, so it's a bit surprising that they simply buff damage rather than adjust MMM proc chance. It seems even more odd when you consider the tweaking of mastery's value for MM via increased proc chance. I'll admit some personal preference there as well, since procs are a bit more fun than simple bigger numbers.

With only some crude napkin math though, it still seems like we'd be using AS due to it being buffed, as well as more potential shots overall because of the lower cost, and of course it being instant.

On the PTR Aimed shot is delaying auto shots, not resetting the swing timer.

So time to dust of your trusty TBC "!Auto Shot, Steady Shot," castsequence then? Hard to believe they really want us to go back there again. Then again it might be intended seeing that the new 2.4 sec casttime would fit all ranged weapons, and condsidering that hidden 0.5 sec AutoShot casttime will be probably gone.

That is definitely good news and a factor that may favor AiS over AS during standard phases. This "only" means delay of approx 1.8 sec as opposed to 1.8 + up to 2 seconds.

I just did a really quick test on the PTR with a premade hunter and removing AS in favor of a hardcasted AiS (still casting AiS when MMM procs) I got ~900 more dps than the test I did with AS and only MMM proc'd AiS.

It looked like WQ preferred to proc on my AS test whereas Piercing Shots was lower. In my hardcast AiS test they switched places with Piercing Shots becoming top 3-4 damage.


Of course the problem with hardcasting AiS is if you have to move your gonna lose all that damage

@ceelion
Did you test on a dummy or was it a raid environment? While AiS benefits from CA, AS does not so the numbers should be a bit off when shooting a dummy.

Yes it was on a dummy. Forgot about that.

This really needs to be tested in raid environment simply because how much Aimed Shot scales with raid buffs:

List of buffs that favour Aimed over Arcane:
-------------------------------------------------------
5% stats
Agi buff
10% AP buff (granted)
4% physical damage
30% bleed damage
5% crit (more PS damage)
10% attack speed
Hunter´s Mark (granted)
12% Armor reduction
Flask
Food
+ all dynamic haste buffs
+ all Agi, Crit, AP and Haste procs

List of buffs that favour Arcane over Aimed
------------------------------------------------
8% magical damage
+ all Mastery procs