David,
I say turbocharge that bitch! It would be really cool and you should see some nice gains. But before you do you need to find out a few things. Go on E55 AMG forums and learn learn learn. Find out if someone else has done it before and what they did to do it. Find out what kind of power the motor will hold, if you need to do any internal upgrades, and ofcourse what everything is going to cost you, and compare that price and expected gain to other options.
Now for the tuning part, you will need to do a custom tune and you need to find someone who does it. I would start with these guys and see if they can do anything for you or point you in the right direction: http://www.eplabs.net/
One last piece of advice. German car people are a weird bunch and I can only imagine that most mercedes people are even worse. You get the ones that will try new things and try to be unique and better and different, but many of them will just tell you "aah why would you want to do that, just get this prefabbed kit for $120000000" because its been tested, its easy and money isnt a concern for most of them. So it sounds like you are going to be a bit of a pioneer.