RENNtech Performance or RENNtech Mercedez is the leading tuner for AMG platform Mercedes. They do all their own work from fabs, body kits, turbos, downpipes, exhaust, installs, writing flash tunes etc you name it, anything performance that has to deal with a car they have the shop and tools to make it.
These guys set records in the AMG, and have been doing BMW cars on based on custom requests. They have just started reaching out more publically in their offerings to the BMW cummunity
So in other words they are not new to BMWs