the 3.6l M5 is a big sedan, while the M-coupe is well a 2 door coupe. You don't say it is a US version M coupe or the newer World Wide M coupe as the euro version was 81HP stronger then the US so this could make a big difference in the race. Also who and how that person drives the car is also a big issue ! The M5 is strong and fast, but also heavy and a little bit dated. The M coupe is small, light, full with modern electronics. Engine size almost same 3.2l versus 3.6 but still the weight is a big factor ! Try the 3.8l M5 and the US stock M coupe and that'll be a very interesting race !