Sub was strong even before T12. I used it for Cho'gall/Nefarian/Omnotron/Halfus/Valiona. It was pretty much equal in DPS at best (this being before the SV buff), but it was great for a lot of fights.
T12 2pc bonus was quite insane, even if Shannox/Baleroc/Majordomo were the only good fights for Subtlety. Assassination was best for Ragnaros and Alysrazor flying I guess, while Combat was for everything else.
I actually played very little of it in T13. Just Morchok and Yor'Sahj (where I focused the boss only), since I never did Spine/Madness.
Sub performed extremely well on Baleroc. It had single target going for it before T13.