@5 stacks the boss has 8.8M hps...I forgot what it had on 6 stacks but someone killed him at that on our server already too.
As for the wearing mask in the cave yes its been tried. I guess we have to kill dreadtooth @10 stacks and see if he drops anything special:P
Wow, the jump from 5.4M (4 stacks) to 8.8M (5 stacks) is a big one. So I guess he's tuned for 16 man at 4 stacks and 24 man at 6 stacks then.