I have only played to +11 with other classes, so my experience is somewhat limited.
When it comes to encountering a pack of adds in a dungeon:
With my BM Hunter I have an INSTANT and no CD, Multishot. Could not be simpler. If I see a pack in the distance I press one button and my group (myself and the two pets) go into Cleave mode. No CDs, no restrictions, no ramp up. Use as often as you like, whenever you like.
With my Prot Warrior, I have 5 to 7 unique abilities, with a CD each, but which I cycle through going from pack to pack. I have Shield Charge, Ravager, Shockwave(Sonic Boom), Spear of Bastion, Thunderous Roar. I also have the bread&butter Thunder Clap, Demo Shout, Revenge etc. So first pack I Ravager, next pack I Shield Charge, next I Thunderous Road, next pack I Shockwave, next pack the CDs are back up. If you see what I mean.
With my Guardian Druid, unlimited Swipe and small CD Thrash. Can’t be simpler.
With my Shadow Priest I have nothing good.
I can do Dark Void + Deathspeaker + Pain of Death. Dark Void on 30 secs CD, and then very random SW:Death procs. By the time SW:D procs, the tank has already finished the dungeon and gone to bed.
A better way is Misery + Shadow Crash + Psychic Link. Much better, except Shadow Crash is a joke of a spell as it requires reticle placement and it is like a Hunter’s trap. By the time the projectile has landed, many times the mobs have already gone. Plus it is very awkward to have to place reticles for bread&butter spells. And it is on a 30 sec CD and there is no alternative to cycle through.
The filler is Mind Sear but at a ridiculous 50 Insanity, and by the time you are ready to Mind Sear, once only, the mobs are almost dead. Tank has moved on to the next pack.
Please get the person who designed the Prot Warrior to spend a few weeks to redesign the shadow priest.
PS: this is not about numbers. You can make Mind Sear kill the whole dungeon in one shot, this is not what I am asking. It is about flow. Please look at the Prot Warrior to learn how it should be done. Or, if you are feeling generous, look at the BM Hunter.