Due to the variables of human reaction time (even if you are best in the world there will be variable amounts of dead time after flays) and latency combined with server I/O you have (even in a best case scenario assuming you are one of the world's best shadow priests and have 10ms latency to the server) a range of about 100ms between the cast time of your best flay and your worst flay, and probably about 50ish ms for any other spell. This randomness has a very pronounced smoothing effect on anticipated haste "sweet spots" if one were to graph haste and dps giving the graph a roughly linear shape. So for all intensive purposes, no, there are no haste sweet spots, however one thing you have to worry about is that if you are in one of the world's top DPS guilds you will be killing bosses much more quickly than the simulators which provide the DPS/stat increase values that you will likely be using. The ramifications of this are that your heroism uptime will be much higher, and heroism typically means that you will be haste capped on everything except mind flay, this vastly reduces the value of haste as a dps stat.
So no, there are no sweet spots, other than reduced value of haste once you get your heroismed GCD down to 1 sec.
There are a few theoretical sweet spots... the 2.75 MF is one of them, but there are many for some advance rotations or recurrent patterns...
But as people have pointed out already, human reaction time and lag kinda make them a little redundant.
You should grab as much haste as possible while maintaining your other stats... if you were really keen you may go for a super haste set if you do nothing but MS during trash (like I do)... but if it's trash its not that important.
It's not really practical to stack haste for the sweet spots that apply in theory.
In practice, all serious business encounters are heavy on movement and few to none are straight stand-and-burns (ie. Patchwerk).
In theory, having a 2.75s MF combined with a 5/5 Imp MB talent seems like a way to create higher dps than 4/5 imp MB and and 3s MF's.
In theory, it's possible to devise the highest rotation cast sequence for as long as fight as may occur.
In practice, you'll need to move at least once every 10-15 seconds, and when you do - that rotation will be lost to the wind.
In practice, rotations are meaningless and priority sequencing is everything. Priority sequences don't care what your spell haste it - 95% of the time your spell haste will be meaningless, and even when it's not your haste sweet spots will assuredly be meaningless (the exception being fights like Patchwerk).
In reality, your vastly better off stacking Spell Power followed by Critical Rating, and essentially ignoring Haste Rating entirely. Critical Rating benefits us in numerous ways, from increasing the percentage damage dealt by our dots, to multiplying the power of our MF, MS, and MB (SW deserves no mention outside of joke threads).
We gain a ton from Crit Rating, even more from Spell Power, and essentially nothing from Haste. Someone is about to quote theory numbers at me, all I can tell you - is that I am the top spriest on my realm, and I am always the top5 dps in my guild for Any fight. What you think works in theory and on paper is not what I experience in reality.
There is one major exception to this, and that is the result haste has on Mind Sear, Haste seems to be very useful for this application - but since I can usually pull 10k dps in major AoE fights, the benefit of haste for the purpose of killing trash really doesn't factor into the big picture.
I destroy trash and I ignore haste, what counts is what happens against single targets (bosses), and that's where Critical Rating destroys Haste rating - that's what you build around - if your goal were to say, farm 50 trash mobs at a time, then sure build a haste set - I kill bosses, I stack SP then Crit.
having a 2.75 sec flay isn't that great anyway. it's not everytime you'd flayed twice that you want to cast MB again. there are all those other spells to cast aswell as moving around, etc.
sure if you stood still and just flayed and MB'd then 2.75 sec flay would be great, but it's just not the case and we know it.
personally I don't aim for haste. I aimed for more crit until I was at 20% and now I just take the gear as it comes. if it's an upgrade great. if my crit goes down and my haste goes up I don't really care.
I've having too much haste is a lot better than having too little as it gives increased time for moving slightly if needed
Haste sweet spots are frown upon by the community who tend to be lagging more than human possible, and thus refrain from admitting the sweet spot exists.
There is a sweet spot, and that's the amount of haste reducing Mind Flay to a 2.75 second cast, as it fits with the 5/5 Mind Blast cooldown.
But it should be worth mention, that it'll not nessary benefit everybody. It benefits me, but the next-of-kin priest can't abuse it. Lag I guess.
You should probably post simulation results before you start espousing such a ridiculously incorrect fallacy. I'm consistently ranked as one of the highest DPS priests in the world and have a 10ms connection to my server, trust me when I say even under the very best case scenario you will not experience haste sweet spots.
Another reason why you should ignore the 'sweet spots' is because gaining additional haste will allow you to cast faster in situations where you have to move around or use GCD's to PW:S or dispel where it is impossible to have ideal dps situations like you can on patchwerk.
Generally speaking you will get into situations where your MB is off CD but you still have part of your MF cast to go... having it finish quicker is invariably going to lead you to cast MB quicker and thus increase your dps.
Back during SWP there was some numbers floating around about various sweet spots and the conclusion was that you should continue to gain haste past the sweet spots because in a pratical situation it can only lead to increased dps.
As Nezoia stated, you get what haste you can from gear, you don't go out of your way to enchant or gem for it. Personally if I can get an item that has superior haste to what I have in crit i'm happy to utilise that piece... an example would be the Arcanic Tramplers http://www.wowhead.com/?item=40558 vs Boots of Persuasion http://www.wowhead.com/?item=40269.
Reasoning is that you get 46 haste vs 35 crit as well as 5 more SP and more spirit.
ps. Someone is going to have to explain how I can make my url's with custom text.
yeah i got a baggins mod but i got stuff stored from my t1-sunwell stuff in thier =/ so its a bit messy even with a bag mod lol. what pieces of gear should i be looking to switch out from my regular setup? i dont have dragonsoul yet, and yet to see one drop. =/