MINI is not/was not "an experimental independant group", you should read up a bit on the history before making statements like that.
These off a good start to learning about BMC/Rover/BMW etc. and the Mini/MINI Marque:
http://www.austin-rover.co.uk/index.htm?ado15storyf.htm http://bmw.bizhat.com/ http://www.outmotoring.com/mini_history.html "Mercedes" don't build the current MINI engine, it's built by Tritec, a joint BMW and Chrysler group venture in Brazil, which was undertaken before Chrysler where part of the same group as Mercedes. According the link you posted above, there's more problem Mercs than BMWs too.
http://www.tritecmotors.com.br/english/index.html What about owners of the Diesel MINI, or the non-supercharged Cooper, they still love the car, but have different flavours of engines. The engine is extremely important, but most people seem to agree the MINI is more about the handling and overall package than the engine alone.
The new engines will also be much higher volume, so the aftermarket tuning opportunities should also increase.