The easiest route might be trading it in and getting your hands on one of the 05 G55 AMGs. It's the last year and they went all out. They put the 469 hp s/c 5.4 from the E55/etc-55 in it, and there is good amount of aftermarket stuff for it. With the price of entry at 20k for a kit for the earlier car, you're probably better off getting the 05 and modding from there. I'm sure you could get 550 to the wheels with the 05 for far less than 20k.

If you want to stick with what you've got, there's no reason not to go for a custom turbo setup over a supercharger. Just find a good shop to do the fabrication and tuning, and consider going standalone depending on the tunability available through a chip. The power you get out of it will all depend on how strong the engine is from the factory, because by the time you reach the level of having to build an engine to take the abuse, you'd also probably be better off with the 05. The hard part is finding the limits without breaking stuff- doing something this unique means having no community to fall back on for information and support.