I don't know yet. I have done a lot of work on muscle cars and the work that you do to those usually are headers, exhaust, rims and wheels. Every now and then bore the engine out, get new pistons, add a supercharger, and put different gearing in it. Is it the same for BMW's.
Installing headers and exhaust is not overly complicated. Wheels you can do yourself. Don't let anyone bore your motor unless his name is Steve Dinan. Supercharger solutions are few and far between. There is a current thread on the development of one. Bottom line -- don't treat this car's motor like a "muscle car"; there is much more going on inside. And do your research to decide what you want to do. Will Turner at Turner Motorsports near Boston is a good resource. You always can consult with the Dinan folks. If you choose Dinan components, a Dinan-certified BMW dealership can be a good choice for the work but get references first. The real BMW performance shops in Virginia are race shops -- Bimmerworld in Dublin, RRT near Dulles, Delta V in Richmond. They focus on race platforms -- e30, e36, e46 -- and don't work with e60s nearly as often but worth talking to (ask for James Clay at Bimmerworld). Fall Line Motorsports in Illinois did, I believe, have a role in building the meanest e60 track car seen here. If you plan to have major work done, plan to travel. If you want a really good mechanic who has done exhaust and various other work on my M5 in Richmond, send me a PM.