As already stated, I don't want CoP (standard) to outperform CoP Dotweaving or CoP Lite, even on standstill fights. Any buff brings the class-balance closer, is nice, and the 25%ish buff to our Mastery, is very welcome, but the idea of spamming MB and MS, with some MF channels while DP is running, sounds extremely boring. Sadly, I don't have access to beta, so can't do anything else than reading up and watching streams atm.
This is not relevant to the discussion at hand, but your memory is failing you. We were extremely shit at the very early beginning of Cata, then in 4.0.6 (some weeks into Cata), they massively buffed MB dmg, and its benefit from Mastery, changing it from hitting like a wet noodle, to becoming pretty much the hardest hitting spell in the game. The 3 next days, we were the most OP class in the game, but that obviously got hotfix nerfed (they changed the passive dmg bonus you got from speccing Shadow, from 25% to 10%).We were ridiculously broken at the end of Cata. (In both T12 and T13, especially with DTR.)
Still, after this nerf, we were the by far strongest caster in the game for all of t11, especially before they nerfed DI. The only class that could touch us on singletarget was the even more OP Arms Warriors, maybe a good MM Hunter or lucky Mage. Everybody was 4 leagues below on any boss where we could DoT more than 1 target. The difference between a good Shadow Priest and a good "whatver class your #2 Dps was", on Valiona & Theralion 25HC, was pretty much the same as the difference between good Aff Locks and your 2# Dps class on Protectors 25HC (yes, the SoO one). At least before extremely short kill-times in SoO scaled Aff Locks through the roof.
We were still the 1# caster spec for most of Firelands, although Arcane Mages with Shard of Woe started outscaling us towards the end.
In DS, we were horribly outscaled by Fire Mages, Legendary Rogues and Arms Warriors, and SV Hunters were simply tuned to do higher dmg than us. If you were beating these classes in DS (single target), you must have had the best RNG in the world, or the ppl playing the other classes, weren't playing properly.
Don't get me wrong, we were still much stronger in DS, than we ever were in MoP, but the SP power lvl in Cata was t11 > t12 > t13, not the other way around. Yes, I am kinda nazi about Shadow and class-balance history.