If any one wants to know how the N64B44O1(biturbo 4.4 litre V8) engine will perform, I believe the B7 4.4 litre Supercharged V8 is the answer.
I believe BMW will however peg the output to a maximum of 450 HP & 450 ft.lbs of torque although I seem them keeping it the 400-420 range for both HP and Torque.
The full capability of the engine will be in the 500 region.
The B7 engines always gets me excited about the forthcoming N64B44O1 engine for the X6,750i and hopefully the X5/550i and 650i.