I five-box 4 shammies + 1 holy priest and eventhough I do not even try to win in 5v5 I still seems to win every 6th game. :P Considering to move from HC instance farming to 5v5 "training" becouse of this success, so to speak.
In my opinion priests AoE heals, buffs, AoE fear is a matter you might want to consider.

Also there is a setup I have been thinking about and might try at some point, 4 x NElf Hunters + 1 x Nelf Priest. This setup would most likely have the element of supprise on its side, especially with cat pets (stealth) and BM spec (obviously).

Unfortunatly I have no experience of multiboxing warlocks so for that question I have no real answer.