oklah I'll give it a go..
here's what i gathered from the internet..basically the best case would be you may just need to fabricate custom brackets to mount the calipers..the worst case would be having to customize/replace more than that such as rotors, e-brake, master cylinders etc...generally what I read is BMW ///M cars have master cylinders capable of pushing far more fluid through to the caliper than our non-M variant. The result of swapping any ///M caliper parts to non-M cars is a very soft and mushy pedal feeling, because the master cylinder isn't capable of generating the righ amount of pressure for the typically much larger caliper pistons present on ///M cars..
before committing to this project, I would try get a sample unit either borrow from seller or mechanic and test the fitment at a reputable brake specialist...that way physically I would know the actual limitations and what parts expected to be replaced/customized/fabricated..I would then also check the price for an ///M master cylinder, whether it'll fit on my car and how much to replace that including labour..finally once total cost is done, try comparing that with other brake options 335i, bmw perf, BBK etc.
But if I were to do it strictly for aesthetic reasons, much less actual brake performance, I may reconsider my thoughts of having the second most important active safety feature on the car just to look good
Cheers n thanks for reading my 2 cents worth of thought
here's some links for reference:
http://www.e90post.com/forums/showthread.php?t=195206
http://www.e90post.com/forums/showthread.php?t=604481 (rear e-brake modification)
