Hey Mr. Z,
I bought my 2003 Boxster S with the 18x10 "carrera" wheels. I run 265/35-18 Michelin Pilot Sport 2s. I have 21mm spacers to push my wheels to the edge of the arch. My ride height is about 2" under stock on Bilstein PSS9s. I have no rubs to date.
In theory, the geometry of a track almost 2" wider should add to mechanical traction. How much, beats me!

Drawbacks? Increased leverage on your springs/dampers.
I can say this though, from an asthetic viewpoint pushing your big ol' skins out to the max looks mighty cool. I have my fronts on 7mm spacers too.
Hasta,
Ike
P.S.: Sharp looking ride you have there.