I think I got the weight from Edmunds and it sounded familiar to what bmwusa had. Here is for the coupe from bmwusa
"And with a curb weight of 3700 lbs., it also has the best power-to-weight ratio of any BMW M3"
http://cache.bmwusa.com/pdf_ff20716a...263a61561.arox
The weight numbers are all over the place what matters is 80 kg difference which is IMO not very noticeable in terms of total weight on a 3700 lbs car.