So I got this
diff plate from a guy named Dan on n54. It was undoubtedly the easiest mod ever - strictly bolt-on, fit perfect. It appears to eliminate the acceleration squirm I've been noticing ever since getting the thing tuned a couple years ago. I thought it would need M suspension parts to fix that problem, but never got around to it because they just didn't appear to be worn in any way. Best 100 bucks ever spent on a car mod. No NVH to it I can tell, so not sure why BMW did not properly mount the diff to start with.
Couple other things of note in the pic. For the guys having trouble getting exhaust lined up, mine was little uncooperative, but that aluminum adjuster brace did the trick. It was only maybe a half inch off, but refused to line up until I added that little doo-dad. The catch can drain can be seen in the bottom-right. I re-used the old DEF tank line there in what felt like an appropriate thumb-nose to the emissions systems. It used to muck up the bottom of the car when it pushed out a bit of oil every so often - not anymore, just a spot on the floor to clean up from time-to-time.