I did it myself on my E92 and am willing to give it a shot on a donor M5 exhaust, I honestly loved the sound on my M3.
Regarding there not being much in the stock mufflers, the Eisenmann is just cans and is much louder, wouldn't that mean there has to be something inside the stock mufflers to quiet the exhaust down?
I'm truly a novice with this stuff, any help would be appreciated.
2013 Silverstone///M5/Executive Package/B&O/20" Wheels/DCT/Black Interior
2013 Imperial Blue///M5/Executive Package/20" Wheels/DCT/Sakhir Orange
2012 Mineral White///M3 Coupe/Competition Package/DCT/Fox Red Interior