I plan on doing the same. I'm leary of changing the front bumper and doing track/autocross. I have a bra and clear bra on my stock nose. Seems like a waste to get rid of right now.
an ebay seller has the sideskirts in fiberglass for the GT3 and Turbo look $300-400. I heard from a previous customer who gave it the thumbs up. I think they have a web page called precision porsche where they list the skirting and GT3 GT2 Turbo front bumpers for higher than the ebay list. Seems like a new business.
I'm leaning towards teh GT3 look
waiting to hear from them on availability since my Carrera 5 spokes are on the way. Maybe we can ask for a group rate or something.
other options
The Aero Kit II in poly urethane are available from Sun Coast(?) for $600-$700
Also sold on Ebay. I don't really dig the shape of those as much as the GT3 but poly urethane is much stronger, although I never had a problem with my old fiberglass skirting on my Miata which sat much lower than my BoxsterS. The only issue was chipping into the 'intake' of the side skirt. The rocks didn't seem to bounce off as easy because of the pocket effect.
I think as long as you have 18" wheels the side skirting should look good without having to upgrade the standard front bumper.