Good thing the M5 transmission swaps right in. I don't know what you are talking about with a 4L30E, but BMW never used a gm trans for the E39 540i. It used either a Getrag six-speed manual or a new five-speed A5S 560Z automatic transmission with adaptive transmission control (steptronic). My buddy has had about 3 supercharged 540s with factory 6 speed manuals at the shop for random gaskets that needed to be changed, but never transmission issues. So no idea what you are talking about.