I have played Sub in PvE a lot. Let me share my views.
- It does better single target damage than Combat. By 1-2%.
- It is behind Assassination on a straight nuke fight e.g. Garajal by about 3-4%.
- Sub aoe is much better than Combat, except if you use AR and/or SB for aoe. Overall because of 30% agi, better energy regen, and SV, it is much better. It is less good at aoe than Assassination though.
- Sub is not the hardest spec to play. Combat is imo, until we are permanently energy capped and back to Sinister Strike spam.
- In terms of just numbers, Sub will, however, get better and better because of scaling and, I think, it will overtake Assassination in the middle of the next tier if no changes are made to either spec.
- However, scaling or not, the main reasons to not play Sub are (i) the positional requirement of Backstab and Ambush - on almost every fight, there are times when you cannot always get behind the boss and (ii) weak target switching because of the SV changes.
In 5.2, I think Sub will be back for some fights, if scaling pushes it over the top as the best single target spec and/or if there are any fights when there are important 10-20 second burst windows.