<<And do SLKs really come with different size rims on the front and back like the seller implies? I never heard of any Mercedes like that from the factory. Seems uncharacteristically impractical.>>
You are uninformed. Almost any MB with AMG wheels has wider wheels in back. As one example, I have a set of 17" C36 wheels on my C320, 7.5 inches wide in front and 8.5 inches wide in the rear. This has been a common optional fitment for at least ten years. >From a pinchpenny's point of view, you bet it's impractical. You can't rotate them. My car seems to be eating rear tires (S-03s) at three times the rate of the fronts. RLE/Seattle