Hartge is also another option which turnermotorsports sells but at a hefty price tag of $1,200. The tune uses the same concept as the JB4 but i'm sure more refined since Hartge is a HUGE bmw tuner in Europe. I rank them higher than even AC Schnitzer...
Current - 2013 N55 535xi Msport SAT
Retired - 2009 N54 335i Msport 6MT
Retired - 2000 M52TU 323i ASA Supercharged 5MT
More photos of my car here