Interesting, I do 1750 DPS on the dummy using carnage spec, but only around 1500-1600 with annihilation. I'll go do a parse and upload it in a few minutes.
Yes, he will throw rocks and stay in place. As Rodech said every tank has a gap closer, so no matter what tank is tanking they should have no problems getting back to Zorn.
Zorn doesn't have an aggro table while Toth is in it's berserk phase. The Zorn tank can run over to Toth and make it back in time to taunt Zorn before the phase is over, especially if they have any kind of gap closer.