The higher output motor (~550hp V10) is supposedly being saved for the M6 CSL version with reduced weight. I believe the additional power is to come from increased displacement to 5.5L, so additional torque too...

Same situation with next generation M3:

M3 400Hp V8 4.0L
M3 CSL 440-450 Hp V8 4.5L with reduced weight

Just rumors that I read in some rags...