I saw a really intriguing post on the Blizz hunter forum the other day, from a highly progressed hunter (6/8 Dragon Soul heroic) who wanted to ask Zeherah (the female dwarf behind FemaleDwarf.com) a question about haste. Specifically, she wanted to know how much haste she would need to bring her cobra shot cast down to 1.5 seconds.
As it happens, Zeherah has included a super-handy haste calculator that made the question really easy to answer. In fact, I doubt the asker would have even asked her question, but I think she probably didn't see the small-text link to "haste calculator" in the upper-right hand corner of the screen. It turns out that you need 33% haste (including the 10% buff and 3/3 Pathing) to bring your CoS to 1.5s, which translates to a minimum of 2264 haste rating for most hunters. It'd be a little less for goblins or hunters that get Dark Intent, but again, you just need to see at least 33% character-sheet haste.
Ok. the interesting bit is why this hunter wanted to know this.
As mentioned before, the T13 2-piece bonus for hunters is amazing. It's actually better than the T12 4-piece or possibly even the T11 4-piece. It is really good. It dumps so much focus into your bar that with it you can probably do a tight Survival DPS cycle with two cobra shots and two arcane shots per explosive shot, rather than three CoS. A 1.5s CoS fits all of these shots neatly within the six second cooldown of ExS.
Just in case you're not sure of that, it's easy to check: 1 second GCD from the ExS + 3 seconds from cobra shots + 2 seconds from arcane shots = 6 seconds total. That is of course the ExS cooldown. Looks good!
Sadly, 2264 haste rating is a pretty steep requirement, especially since you're still going to want to reforge for crit first. You absolutely want critical strike rating on every single piece of your gear, and you never want to reforge out of it. Then you have to make sure to reach 8% hit, and there's little enough hit on T13 gear that you'll probably end up reforging into it. Finally, you don't want to go any lower on mastery than a 5:1 ratio of crit:mastery. I actually don't think you're likely to run into a problem with this last one (if you have 600 mastery, which is not much at all, you'd need to go over 3,000 crit rating before you were worried about it), but it is something to keep in mind.
Basically it's just going to be tough to reach 2300 haste rating even in 397 gear. It might be more reachable in 401 gear, but by the time you've reached the point where you have 401+ in every slot, you've completed all the heroic modes anyway, right?
All is not lost, however!
There are a lot of haste effects in the game right now. Rapid Fire, Bloodlust, The Hungerer, Arrow of Time, and Starcatcher Compass will all bring almost anyone over 33% haste. So if you're playing Survival with the 4.3 buffs, I'd recommend having some way (probably Power Auras) to track all those effects and procs so that you can know when you can switch to a 2 cobra/2 arcane DPS cycle.
And if you don't have T13 2pc yet, get working on it! Get into those LFR queues and start rolling need on tokens!
Oh, editing in a bonus question that just occurred to me: do you suppose that this DPS cycle might mean Kiril is not BiS for Survival? It is a mystery!