I might have been confused, but are we talking about solid bushings between the diff and the subframe, or solid bushings between the subframe and the chassis/unibody?
I would imagine solid bushings between subframe and chassis would transfer a lot of shock and vibration to the cabinet which might not be desirable for a street car.
We are all speculating here and the culprit might just be that these nuts are not torqued to spec on a Friday afternoon in the factory in Germany. :P However if these bolts are indeed giving out because of movements of the diff, wouldn't these movements/stress be absorbed by the ear/arm (the mounting tabs) of the diff and the mounting point of the subframe if we were to use solid bushings between the diff and the subframe? I am no expert on this subject though so just guessing.