The M5 has a turbo V8, so seems natural for the M3 to have a smaller engine. Even with a six cylinder though, it doesn't mean the car would have to be down on power.....just look at the GTR.
BMW seems to have greater differentiation in power/speed between the M3/M5 than do AMG models. With the M5 having 550bhp, the next M3 might even have less power than the current C63. Just how it is....still plenty quick for me.