There is a Priest who plays 4, seems effective and you hit the breaking point on mana spent vs mana return at 3 Shadow Priests with good gear, at that point you have unlimited mana - as long as your enemy still has hp.

I've taken 3 to 61 but stopped there, maybe it was burnout or they just didn't 'feel' as poweful as my 3 mages.

4 Hunters 1 Shaman would be nice for PvE but would get demolished in PvP.

4 Warlocks/ 1 Priest is a nice all around pick.

I don't think anyone has touched 5 druids, most likely due to Melee classes are a nightmare to multibox and they're offensive spell casting is not mana efficient at all.