My guess would be 450 horsepower in a triple turbo charged inline 6. Basically, taking the n55 engine and pimping it out.
Result after less weight and more horse power:
0 to 60 - 3.9
1/4 mile - 12.0
I hope BMW continues with the double clutch option.
MPG - 20 city
It will probably sit at 1500 rpm at 80mph.