I started off with the standard OEM 18" wheels as well and decided to upgrade. I got replicas from
http://www.wheeldynamics.com/ . No fitment issues they are made identical to the Porsche wheels. Wheel Dynamics is also great to work with and the wheels seem to be high quality from my experience. I would recommend them.
I'm very happy I did the upgrade, in person the 19" wheels make a huge difference in the overall appearance of the car.