I think I will do solid subframe bushings next. Thanks for the suggestion.Go with solid metal subframe bushings. No added NVH and the new bmws come with solid from the factory.
On the rear diff if you go solid you will get diff whine. So your call on solid vs m3 or poly. I went solid and it does not bother me but there is gear whine. I would suggest a ride in a car with solid so you can hear it.
I would start with subframe and diff and if that does not fix your issues look at the rear suspension components.