From what I read sometime ago, the engineers stated that the E9X M3 will not weigh more than the E46 M3.
The E46 M3 weighs 3415 lbs
The E60 530i weighs 3472 lbs
The E60 550i weighs 3803 lbs
The E60 M5 weighs 4012 lbs.
I see the engineers pulling this off.