I have had a 530 with M5 25mm front and M5 18mm rear which was quite nice. I had M5 wheels with 235/45/17's all around on that car. Handling was much improved and no complaints.
Present car is 535 with M5 25mm front and M5T Nurb 20mm rear bar. The larger rear bar makes my car very neutral. Especially since I am now running staggered wheels 235/45/17 front on 8" style 32 and 255/40/17 rear on 9" style 32 BMW wheels. My suspension is Koni front shocks with Sachs kit otherwise.
The rear bar can be bought at a very reasonable price through BMA (www.bmaparts.com
) and they drop ship from Germany to you. Install can be done without loosening the exhaust. Fish the old bar out and new one in without the end links. Spray end links with silicone and rubber hammer them onto the ends after the bar is in place.
Spring Valley, CA