I don't agree re: why bmw used a CF roof. They used one on the CSL e46 and the previous M6. Plus its more like 20 pounds vs a steel roof and 45 compared to a moonroof but location of weight is everything. For example replacing the seats to lighter weight have virtually no real performance effect from a handling perspective given they are at the center of rotation of motion and really have negligble effect.
The lighter roof is still 20-45 pounds lighter and every 40 pounds counts no doubt. More important is it really is the highest aspect of the car and using the CF effectively lowers the car and although those small increases in performance may not be something obvious alone, when you take the 10-15 little things they did such as use plastic for front a rear bumpers/body work, aluminum pieces, aluminum brake hubs and floating rotors etc, all of that adds up to some real performance value.
In 2001 when the CF was being tested on the old M6, they were not even thinking about the I3/8 series and the e90 m3 was simply already going to take the CF given the M6 had it so it was a decision effectively made in 2001 with the M6.
Plus it looks great which is key!