There's a lot of talk about people over in the regular e90 forum changing their sway bars.
I have already changed my rear bar from the 13mm bar that came with the sport suspension to the 14mm bar that comes with the M euro package and I definitely noticed the difference.
There's plenty of talk about what works and what doesn't.
There's some that say that the 20mm bar makes the car feel much more balanced but there are others that are saying that when tracking the car, there will be less traction since I'm driving with a peg leg.
Is there anyone local that has changed the rear sway bar in their M3?
If so, I'd like to install it see if it works for myself.