[Perfectlap]

17's look best on a car with no aero. But a Boxster with aero looks the closest to the '93 concept car that knocked the socks off everyone who saw it at the international launch. But since Porsche nickel and dime you for all options they sold them stripped of aero, barebones interiors, small wheels, etc.
And a Boxster with aero looks best with 18" because the wheel well space gets larger once you add a bumper like the 996 GT3 that sits much lower, and is longer. Which means you then have to add skirting too in order to balance it all out and keep it from looking like a high-sitting arrow.
Aesthetically, wheel width matters too. A smaller wheel needs spacers to fit a bit more flush with the fender edge and I'm not a fan using wheel spacers because of the beating on the bearings. I use them during the winter months on a set of replica beater wheels but my mileage is substantially lower during these months and I'm not taking any hard turns. And winter tires with their hard rubber really cut down feedback so its more cautious driving all around this time a year. In fact on well below freezing days I take my bicycle out (now with spiked tireds) instead of subjecting the engine to wear.

A factory 5 spoke Carrera is the best wheel for a Boxster with or without aero, and it's a 20lb. wheel. Too bad they're crazy over-priced for a cast wheel. You're better off going with an 18" OZ with at least a forged flange. Forged everything gets real spendy.