I know I'm looking forward to my shadowpriests...

I'll probably run 4 of them. For PvP I'm guessing you can kill in about 2+ GCD with two mind spikes and an instant mindblast (I know that's three GCD, but since the last one is an instant you're killing in slightly over 2). If that doesn't finish them off then just follow with a nice round of Shadow Word Death, most of which will not get the killing blow which means their cooldown is instantly reset.

I think the real question is which class is the most OP when boxed. The most OP solo class probably loses many of its strengths when having to coordinate multiple toons.