Succubus is good if you need knockbacks, so Shaman, Immersius immediately spring to mind; also worth considering for Garrosh as the knockback will interrupt MCed players' casts. Also good if there's a decent amount of movement/target switching.
In terms of DPS, the difference isn't that significant unless you're able to make a lot of use of Havoc Shadowburns where Sac may be preferable for damage alone. Depending on your setup though, utility may be far more important to killing the boss.
If the adds die before you can SB them, yes a pet is good.
Taking Galakras as an example, the second wave has low health adds + 1 tidal shaman that has around 4 times more health. Because of that and AoE, the adds close to the shaman will be on low health while he is at around half. At this point you Havoc the shaman, 3x SB on 3 different adds, they die and you get the embers off them and the shaman is in execute range so you can dump another 3-4 SB's onto him just killing him.
Let's say your average SB will be around 700k. You just did around 6 SBs in a couple of seconds and that's 4.2mil damage. 15% of that = 630k damage.
I'll go ahead and say that a pet can't do 630k damage in around 6 seconds.