Did your search include BMS?
These guys seem to have some experience:
http://www.burgertuning.com/BMW_wheel_spacers.html
I've been toying with getting 12mm back 10mm front on my stock e90 with sport suspension.
Might try it after swapping for the performance suspension.
Those rear wheels are WIDE, but oddly not as wide as the wheel arches. Are the rear quarter panels the same as the M3? Cost savings with parts sharing?