I simmed you with 50k iterations under the settings I do all my comparison sims at and I got similar results so I wouldn't worry about that. However in future I'd highly suggest you do 25k+ iterations as 1k is going to be very inaccurate.
The T15 set bonuses, meta gem and cloak RPPM procs are still all increased by Haste. Source.
Hmmm okay i guess ill try reforging haste and see what happens...
edit: I also feel like im doing something wrong on SimC, it says I sim 253k dps but on Malkorak i did 275k. Of course i expect some variance but not that much? I leave the options all on default settings and have all the buffs on so I dont know what I could be doing wrong..
You likely didn't have settings done correctly to compare to that specific log, mainly the fight length. I sim you at 257k with a 360-440 length fight but with a 250 length fight (only 16 seconds shorter than your Malkorok kill) I get 273k