I have a 06 standard with basicly the same wheel. The train has the cool black center of course. I'm having a hard time keeping the wheel clean too. I'm thinking of having it chromed over the winter. I hope it is easier to keep clean.

 

Chrome it. A much smoother surface that is not porous.
www.chromethathog.com

 

If you like the black center and are willing to go the price why not have it polished up really nice and the center powdercoated black or dark charcoal gray. Polished its easier to to clean and the powdercoat is better, tougher, and easier to clean than the original paint.
If you do have it professionaly polished I would suggest you get it clearcoated. Then you just have to use something like Simple Green and a hose to keep it clean.