i have an e55 & i would say just adding the c63 back boxes is the easiest & probably the best exhaust mod. The sound is almost the same untill you give it a kick up the back side then its much better.
I have a couple of comparison youtube vids. Just search my threads.
^ Not the easiest of jobs.
Good luck with getting anything from VRP. They have an awful reputation on the US forums for slow/no deliveries and zero customer service.
If I had my E55 again, I'd look at MBH headers.
May also be worth a call to Acid at MSL Performance to see if he has any Eurochrged headers for the 55.