To hold off the upcoming F80 M3 and F82 M4's 425 horsepower S55B30 motor it is likely just enough. Keep in mind the W204 C63 was rated at 37 horsepower more than the E92 M3 and had a larger motor but due to the M3 being lighter and having an optional dual clutch transmission it was a driver's race with similar acceleration figures.
It looks like history is set to repeat itself as the W205 C63 AMG will not have a dual clutch transmission option and is likely to be heavier than the 3306 pound M3/M4. The W205 C-Class is lighter than the W204 but we will have to see how much weight Mercedes is really able to cut from the C63 AMG. Even getting into the 3600 pound range sounds optimistic.
This next generation battle is shaping up similarly to the previous generation with the C63 having the bigger motor and being heavier whereas the M3 is lighter and relies on efficiency technology such as its dual clutch transmission. The wildcard is another performance package from Mercedes which BenzBoost already confirmed. BMW this time around with a turbo motor though will be able to respond with a performance package of their own as the do on the M5/M6 models.
Brute force versus efficiency? Something in the middle? We shall have to wait and see.