That really isn't the case, having previously owned an e90 m3, I can say there isn't that much shared between that and an e90 335i (engine aside), there almost different cars
Pretty much every body panel is different, the only ones that were not, were the boot lid and front doors.
Undertrays, arch liners, suspension arms, sub frames, diff, brakes, suspension, the list of upgrades just goes on and on.
It'll be the same for the G8x.