I wrote this about three weeks ago in another thread on this board - suffice it to say I AGREE! We need to campaign for an 18"-rim-friendly BBK for the track.
"I did find myself thinking about Stoptechs the other day. When I got the car, my plan had been to put them on as early as possible. I had them on my E39 M5 and they were super. However, I eventually concluded that marketing overcame common sense in their product design for the E9x, forcing me to stay with the stock brakes. Not a bad choice, really - they're working fine so far.
The problem I have with the design of Stoptech E9x fronts is that they have 380mm rotors and 6-piston calipers. From my 400 pounds heavier M5, I know that 355mm rotors and 4-piston calipers will stop this lighter car just fine. Stoptech's choice of the big rotors and calipers force you to use 19" rims, sharply restricting your available track tire choices. The few really good extreme-summer and R-comp choices available in 19" will set you back between $400 to $600 more than 18's at every tire change. The sad part is that there's no performance benefit from the larger tires, larger wheels or larger brakes - they reduce your options for tires and brake pads and they make driving more expensive - that's it.
The perfect track-rat package for the E9x brakes would be 355x32 rotors all around with ST-40 calipers. The range of available pad materials is vast, they're a breeze to change, 18" tires and wheels will fit, and nobody will ever miss the two-tenths of an inch in rotor diameter.
Sigh. How hard can this be?"