285's are wider, heavier, may look better (filling the fender thing) but depending what size tires you run up front, may contribute to a feeling of more understeer. It's hard to break loose a 285
Factory sizes for 18" wheels on the Boxster were 225 front, 265 rear if that helps.
a 285 is generally 20 mm wider than a 265. 20mm is just less than one inch (25.4mm). If you have lowered the car, it is conceivable that the 285 will rub on the rear springs/strut.