So, on the original question. I have been gathering information on ST and AP BBKs. As far as I can tell both kits are from reputable companies, and both are quality products. Parts and repairs are available for both. ST is much more popular, and has glossy paint. One area that I want to understand better is the design of the bbk for our cars, I have become curious since reading the ST white papers on brake bias and "balance". Marketing info IS aimed to a broad audience.
I enjoy tracking my car, and the fucntional aspect is most important to me. What really intrigues me is which kit has the best design specifically for our cars?, which company did their engineering work? Which kit is best designed for track use? Our cars are front brake biased as all general consumer road cars, which kit does a better job to optimize bias?
Stoptech claims they do this, that say they "improve on the OEM brake bias" tyically by moving it back 15-20%. I was able to validate this claim doing some basic math to come up with total caliper piston surface area. However, the real question is whether the design of the caliper change actually do anything to affect brake bias. I understand our cars have electronic proportion valves, so I would think the caliper piston sizing will not do anything to affect brake bias. A BBK will only help in managing heat using a more efficient caliper design, larger/better designed rotors, better materials, etc.
I know I am over simplifying and have limited knowledge on this complex brake stuff, but it seems ST brake bias and balance system claims may not apply to our e90s.
2011 e90 N55 (now 100% stock) & e36 2.8L SCCA ITR race car