depends if the slk500 seats are actually leather and not MB-Tex. (btw, never heard of a slk500, slk55 or sl500 maybe?)
If leather, check that it isn't coated.. If the seat is full of perforations, normally that is a indicator with MB's that the seat portion at least is full leather, and less likely to be coated.
Then yes leatherique will work well.
I use leatherique on the Benz and Leather Masters on the BMW.