Blackrock Depths - 53(hard), 57(challenging), 60+(easy)
Lower Blackrock Spire - 55(hard), 58(challenging), 60+(easy)
Dire Maul - 58(hard), 60(challenging), 63+(easy)
Scholomance - 57(hard), 60(challenging), 63+(easy)
Stratholme: Live - 56(hard), 59(challenging), 61+(easy)
Stratholme: Dead - 55(hard), 58(challenging), 61+(easy)

IMO...

[edit] This is with a priest+4 warlock group. I'd assume similar results with other non-optimal (i.e.: non-holy-trinity) groups, like hunters+pally, shammies, etc.