Im not much of a hardcore raider, so my gear basicly comes from T14 normals, T15 LFR's and Valor. Atm im about 2% over the hitcap, with reforging all Spirit and hit away from my gear. Now, i still have the 2 setbonus from T14. If i was to get rid of the 2 Setbonus, in order to gain about 700 Crit/Mastery/Haste and getting my Hit back to the cap, would that be a DPS Loss or Increase? And by how much?
just sim it on simulationcraft (test both settings*, and stick with the one that give highest dps if the difference is significant), depending on your ilevel the set bonus will be worth more or less, so there is no "easy answer".
* afaik there should be a way to export an askmrrobot profile to simC, so that you won't have to do all the changes ig (which might be slightly expensive) or by hand in simC (which can get tricky)
I'm assuming you have wushoolay's final choice as a trinket. I was in the same situation last reset, though my ilvl might be a bit higher than yours (including stats, but the idea is the same). I had the new tier hands and shoulders, a 509 chest, and 496 helm and legs (so 2x T15 and 2x T14). Replacing my 496 helm with a 522 set item helm, which would break my 2p T14, and upgrading my 496 set legs with 502 legs that I still have in my bags resulted in a 1600 dps loss for me, on a tank and spank fight. I've mixed all possible gear and trinkets around that I have, done a shitload of sims, and always the same result: I keep using the 496 helm and legs and keep my 522 helm in my bags until I grab a 4th T15 set item.
While the gap might be smaller with lower gear levels (I'm 1.6% above the hit cap now), the same logic might be applicable to you. The safe way is of course to use simulationcraft, as suggested earlier.