Re: Wheel size
Didn't some SLKs come with 16s ?. The key is to maintaining the same tire diameter. I have thought something similar in some of the frog stranglers we get around here. Suspect a narrower tire with very good water dispersion might be better particularly in the back but the Contis have been pretty good.
OTOH the 245x70x17 Goodyears on the heep are at about 6/32nds and I see the TC light come on quite often in the rain. Think it will get a set of Michelins before the next monsoon season (now coming into the glorious weather Floridians put up with summer for).
Back in the day I ran 27" 12.65x15s on the back of my Corvette which was a bit lighter than a XF and had about 470 hp (SAE Gross). Today that would be about a 325x50x15. They were useless in the rain.