Amanda,
Those tires and wheels are within the acceptable range of rim width specs published by the Tire and Rim Manufacturers Association. Unless you are experiencing a handling problem with those tires, don't worry about it! A visit to
www.tirerack.com can give you options for future tire purchases ... once a set of tires is displayed for a particular brand, click on the 'SPECS' button to bring up specifications for that entire tire line.
As a side benefit ... since Porsche speedometers are notorious for reading 'optomistic,' or up to 10% higher than actual speed, the net effect of the taller rear tires may be to give a more accurate speedometer reading.