Everyone who would rather have a sunroof with an extra 50 lbs at the top of the car should either get the convertible (to truly experience top down motoring at the penalty of some weight and structural rigidity) or get a 335i. The CF roof will make a huge difference in handling and feel of the car. For those who won't notice the difference, you probably won't notice the difference between an M3 and a 335i's handling either, so save $15K and get a 335i. It will be almost as fast (with Procede piggyback ECU = 380 HP) and handling is excellent for the average driver.
Driving sideways: It's not faster, but damn it's more fun!