I just read a recent article that compared the M5, the Audi S6, and E63 AMG Benz. The M5 was ranked 3rd.
Rumor has it that Cadillac will be introducing the new 600 HP Corvette SS supercharged V8 engine in a special edition CTS. Then again, there's the GT-R.
With multiple makes upping the ante, I hope the new M3 is up to the challenge. Perhaps a biturbo V8 version wouldn't be unreasonable to consider.