Oh...good to know. Guess I will continue to run DOT tires@C7 yeah the cayman has a roof rack mount under the rain strips there. Bolts right in and those things can hold some weight. This is pretty common on a lot of euro cars.
On the s550 there is just that little piece of trim above the windows for the rack to attach to. That might be ok for a snowboard or skis but I am thinking its probably not the safest solution for ~160lbs of wheels and tires.
Can't you fit them in the interior?
if you wanna check out my rig, drop me a line (it looks like you're in the portland area, too). beats lugging your wheels on the roof!