Grimoire of Supremacy is what I am spec'd into.
My buffs were: Dark Intent, Enchanted intellect(320int), and Alchemists stone.
The only debuff on the dummy was: Curse of the Elements.
Each pet hit a target dummy (not the Raider's training Dummy) for 5minutes.
Shivarra - 2,295,828 damage total, 7596 dps.
Observer - 2,282,803 damage total, 7595 dps.
Voidlord - 2,196,722 damage total, 7617 dps.
Fel Imp - 2,307,977 total damage, 15,926 dps.
Here is another test, same as above but 30seconds, instead of 5minutes.
Shivarra - 284,476 total damage, 8441 dps.
Observer - 265,440 total damage, 8165 dps.
Voidlord - 241,082 total damage, 7641 dps.
Fel Imp - 343,092 total damage, 16,728 dps.
*My stats are: 12,082 Intellect.
18,946 Spell Power, 7.23% haste, 14.97% Hit, 12.19% Crit, 43.68% Mastery.
** My spec is Destro
***This is for single target PvE fights, not AoE. Not for PvP either.
So in closing, choosing a pet is situational when it really comes down to it, from what I have found at least.