Ok, this thread is hilarious!! To die hard trackers, I agree, big rear sway can cause problems. For "have fun" street drivers, the M3 rear bar is great. I've even had an H&R rear bar and it was fine, on the street. I never crashed and had a car that felt sharp.
My last E90 had BMW Perf Springs, koni yellows/whites, and M3 front and rear bars and the car was really fun on the street. kinda go-kart-ish.
I now have a stock ZSP E90, and it's pretty dull. I have dinan spec rear shocks waiting, just need front yellows and I'll go from there. May do sways but I'm hoping not to have too. Not as invested in this car.