I'd give a thumbs down to the 19" size. I think you are looking at a really rough ride and risk of trashing the rims with potholes. I personally don't even think manufacturers should make 30-series tires, that's just not enough sidewall to be safe.
Since it sounds like you'll be taking delivery with the 18" wheels on, I'd put some miles on before deciding you want something even stiffer. There are lots of really cool looking 18" options for the Boxster, check out
www.WheelEnhancement.com, which--lucky you--happens to be based in So Cal.
Congrats on the car.
-Charlie