Not sure. I put KW's on my car, and thought about going to heavier sway bars. The rear bar isn't very thick. After a lot of research, I was lead to the conclusion that a thicker rear sway might be a problem, due to wheel hop and a situation where the back end might break loose very suddenly with little or no ability to correct.
I also got a lot of opinions that the lack of an LSD was the weak link in rear end of my E92. So now I have a Quaife LSD and the KW's. Not sure is I want to do any more work on the suspension. The coil-overs really flattened the cornering, and the LSD allows me to put the power down both coming out of corners, and off the line. I could make a lot more changes with M3 bits, etc... I am very satisfied with the handling, and I think it would be quite expensive to get minimal gains at this point. I don't know how much improvement is to be gained at this point with sway bars, and the rear sway is quite pricey to install. That being said, I have probably already covered the most expensive mods to be made to the suspension.
JB4, BMW Perf. Exhaust, DCI, Strut Brace, KW Series 1, Quaife LSD, Black Kidneys.