I believe it wasn't until the 98 model yr that Porsche beefed up the suspension to permit the Boxster to handle 18" wheels. 19" wheels were never recommended for the 986 cars, but were options with the 987 debut in 2005. That being said, I doubt you'd have any trouble if you don't drive the car too aggressively. Just look at all the cars out there with 22"...doubt they're approved for double deuces...
