I notice a lot of haste discussions in here, and i wanted to give my 2 cents. You might see a 200 dps increase in Simcraft with error margain making it non dps upgrade to stack mroe haste. You need to remember that simcrafts simulate you 10.000 results of tries, where you see a 0,0000000000000001% higher chance on your RPPM trinkets to proc. Keep in mind you "cap" your highest burst potential lowering your mastery, because mastery just scales the best with dots. If you put one dot line on a mob on Horridon it does more damage with mastery stacking then haste, notice how you go HAM on Prim with mastery buffs? Simcraft is 'overvaluing' haste quite a bit.
In average you will kill a boss 10-20 times pr tier, that's 9980 less tries then simcraft average, so your 0,15% more rppm trinket proc really doesnt matter! It's RNG, its luck and even with my Thunderforged HC 2/2 Lei shen trinket, last raid i had 3% uptime on Jinrokh and 4% on Mag. I dont think anyone wants to kill a boss 10.000 times to do overall 200 more dps!
So you need to look into what you should benefit from. If you sacrifice doing a few thousand more on your top DPS parse for doing 200 more DPS overall on 10.000 tries, with how RNG the game is at this state, what would you choose?
That being said, going agony / UA haste caps > mastery should work out, full BIS + 2/2 upgrade its going to be hard to get rid of all your haste anyway!
But those saying "go all haste", i really disagree and i tried it out myself. Most fights as Demonology the heavy mastery just shines too. But a good guideline is the 9778 vs the 13k breakpoints, but going for the 13k breakpoint wont increase your DPS if your shit doesnt proc
we are talking minimal benefits being within error margains and simc is overvaluing haste to such extend that the 20 times you killl a boss you WONT possible be able to justify and see it work out.
Watch out with the haste stacking, pray to the RNG gods and go ham!