I would recommend a K&N air filter (cheap 3%HP gain), wider tires. such as the Nitto 555 285/35ZR18 for the back and Nitto 555 255/40/18 on the front. That will help you hold on for those 5 second 0-60 runs. They will still work with your stock rims.

To me the 2.82 axle is for big top speed, which is cool in Germany, but can ruin your insurance bill here. I bet that if you changed that rear end to something like a 3.11 or so, you would be as fast as anything out there. Those slightly beefier tires would help then. I am no mercedes expert, but I have test driven that car at Mercedes of Oakland and it is perfect. I don't think I would change a thing due to resale value, etc. You are one lucky guy to get that car! It is a real runner and the best looking car out there, or one of them.