Originally Posted by The CSL
If Porsche can get 415bhp from a six, why can't M?
Because they have more displacement to work with. The S54 is already oversquare so you can't really stroke it and maintain 8000RPM redline, and there's not enough space left between cylinders for a bigger bore either.
911 GT3: 415 / 3.6 = 115.3
E46 CSL: 360 / 3.2 = 112.5
BMW has stretched the motor almost as far as Porsche has. Personally, I am glad they went with a V8 rather than just give us the motor from the E46 M3 CSL.
Oh and yes BMW could build a bigger I6 (they used to have them - Google Senior 6 or E34 M5), but they are too long to fit in the engine bay while meeting modern crash regulations.