OK, found my own answer regarding the diff here -
http://www.bimmerforums.com/forum/sh...-mount-failure
Looks like the monocoque was only ever meant to take a lightweight 140hp motor and anything bigger can stress the structure to the point of catastrophic failure.
But hey, at least you can boast to the other patients in A&E that your car was going 150mph when the back wheels came off...