Torghast is balanced around every spec individually, as in there is no bad spec and all of them are exceedingly good.
From my tinkering, in BETA it seems unholy DK and MM hunter is the strongest and fastest classes, as MM hunters 100%+ movement speed and the same goes to DKs.
Unholy DKs get a special bonus as they also get bigger, so you end up being raid boss sized and super fast.
As for kill speed, any class/spec if they play their cards right can end up oneshotting everything, its usually the first few floors that are the hardest.