I run 17"s on the track, 225/45/17 and 255/40/15. Stock sizes for Boxster in 18 are 225/40/18 and 265/35/18, not that much more rubber, and they are much more expensive.
In Hankook RS3, I can't really go any wider with 17's, and not really that much wider with 18's. Same with a tire like Nitto NT-01, can't get any wider in 17's and the wider 18's are way wider like for GT3 type.
I think Hoosiers and the like have more options for getting wider and custom wheels may support those wider widths.
For me it is mostly cost I think, way cheaper to run 17's.
Others may have some feedback and maybe there are more tires available that I don't know about
