With the e89 having a rear axle based on the X3, there is no way bmw could transfer a contemporary m3 (e90) or m5 (e60) rear axle. And the e46 rear axle with diff wasnt made anymore (not in those numbers.). So no realistic way for bmw to put a rear axle under it with LSD and handeling more power.
And I dont think bmw would bring out an m car with huge power but not the handling to match (the press would kill it....).
Really a shame because they raced the e89 so succesfully, winning everything they entered.
Obviously the e89 gt3 car has no technical resemblance with the e89, but why spend all that racemoney and not being able to market it into a consumer model.....