If you're getting an M3, you're getting a car that you can use in the real world and is fun to drive.
If you're going to the track:
a) 1/100ths of a second matter in every turn...count up how much time you save in a turn, multiply by how many turns and the number of laps...it will get into the seconds
b) You're getting the wrong car for the track. It will go around quick, but a CF roof will still not make it faster than other cars better suited for the track.
So, if a carbon fiber roof is that much more enjoyable to you, go for it. If you prefer a moon roof, have at it. I went for the convertible and am not complaining about the extra 440 lbs that adds...