Originally Posted by k3vkoh
I always thought that M is the tuning department for BMW, AMG for MB, just like TRD and Nismo.
Except BMW doesn't have an M line of parts, except for what's available on the M cars. The others you mention have performance engine, suspension and appearance parts up to racing stuff. Cams, supercharger, brakes, etc. are all available and, if installed by an authorized dealer, are covered by the vehicle's original warranty. That's a big benefit.