Whatever wheel/tire combination you have you must not exceed the maximum tire diameter or the maximum tire width. Sounds like you have exceeded the max diameter and when the car suspension auto adjusts or takes a bump your tires scrape. I dont know all the math but on the net you can find how to calculate a wheels diameter from its 2xx/xx R1x size. Use the AMG wheel sizes from 00- S Classs as you starting point to find the max diameter/width or an MB dealer. You can have 20s on the car but you have to make sure the wheel can fit a tire that has a low enough profile to not exceed the max diameter and that the tires that fit the wheel do not exceed the max width. Try here first
http://www.sumitomotire.com/tiretips.asp You can get the tire catalog here
http://www.sumitomotire.com/SumitomoCat_bars_blue.pdf Use the 16 stock tire size and in the catalog and when you find it you will see what the diameter is. You can use that as a guide since if the wheel diameter is exceed your speedometer will be off. Look for 20'' tires, if they have them, and see if they fit you wheels. If not the first page tells how to calculate tire sizes. In the meantime you may want to use the raise car button on the dash to give you another inch of clearance, the drawback is at highway speeds it automatically cancels itself.