I'm a 550i owner and I had the pleasure of hearing an M6 today as it was delivered.
It sounds F'n incredible. At idle it has a loud Brrrrrrr sound and seems as if it was idling fast but I'm quite sure it was idling right where it needed to be. When they blipped the throttle it almost sound like an F1 car.
NO question this will be my next car if still available in 18 months.
No sunroof, carbon instead. I believe the front fenders are of a composite material as well.