CJ thanks for your input.
I want the bigger rear bar to help the car rotate during autocross. Plus I feel like the smaller (e90) m3 bar with the larger (e93) front M3 bar will help to keep the car from being overly tail happy. That, along with tc/dsc and a hell of a lot less torque than a 335i should keep the rear from stepping out too bad. In addition, the rear sport springs are a lot less stiff than the rear springs in an m3.
All told it's just a guessing game and I hate that. There's so many variables and I would definately rather rely on experience of others.
But I agree, since it is such a bitch to swap bars I would like to get it right the first time.