Some justification for the weight numbers in the OP.
Audi R8 GT vs. standard R8 weight savings:
This was a big thrust toward weight reduction for this variant, more so than a big power or torque bump. They achieved:
-Seats: 69 lb
-Lightweight carpet: 17 lb
-Thinner windshield glass: 20 lb
-CF hood , bumpers and side "blades": 29 lb
-Battery: 21 lb
-Intake and sound insulation: 11 lb
-Carbon ceramic brakes: 20 lb
This already totals almost 190 lb, well above the upper limit of the number I've estimated for the M3 reductions. Of course I'm pretty sure the next gen M3 is not going to get CC brakes, but either way the numbers I have posted are totally realistic.