Thanks for your input, Brad! I'm not planning turning the Boxster into an SUV, however.

My coilovers are adjustable (see first link in initial post) and I have plenty of thread to spare in order to compensate for the increased "overhead". And although my parts have a Porsche Motorsport part number they're identical to the ones on the stock, road going GT3's.
The problem is rather which hardware I need to add. It
seems (hard to know for a fact since they're mounted on the car) that the inner diameter of the springs is roughly 70 mm. This would indicate that at least the Patrick Motorsport/MODE part would work since it's designed for the Bilstein PSS9 and its 70 mm ID springs. Does that sound reasonable?