A Duramax with a box that has a good tune can easily see 400 HP. I've never seen one with 600 HP w/ just a box though...
It is no uncommon to see a Duramax in the 13s, or even 12s. There are even a few running 10s in full street dress and full sheet metal.
He'll have traction too, with an upgraded tranny he can launch hard in 4WD.
I wouldn't count the truck out by any means... but keep us posted!