I actually think the M4 will be the high end coupe everyone has been wanting while the F80 will be a tamer version. A smaller M5 lets say but lighter too.
I have no evidence other than BMW really spent big on the M6 compared to the M5 and the M5 continues down a different path.
I just cannot see how the car gets lighter by all the claimed experts when BMW carbon fiber technology is a long way away.
Only way I see it is an aluminium frame like Jag but that totally changes the way the car is built and I cant see that changing either.