Those are the standard 18" Boxster tire sizes, and lots of folks have run the 18" Cayman S wheels on their 986 boxsters, which have those same widths and offsets. Some run a 5-7mm spacer in the front for looks with that combo, but I don't think it is required. Search for "Wheel" threads or for folks that are running the 18" Cayman S wheels to get a confirmation from someone that is running that size. In fact I think someone on here recently got a set of 18" 987 wheels and is successfully running them.
Edit: looks like the other forum member I was thinking about has posted to the thread with pictures
