When I first got my boxster a few months ago, a friend was telling me about those cupholders.  The first versions were all plastic and prone to breakage.  Later versions have some sort of metal reinforcement.  I think he told me they were $50 each from the Porsche dealer.  I settled on the a set from 
www.ultimatecupholders.com.  I'm pretty much restricted to 12oz can when I use the cupholder on my side - my right leg hits anything bigger.  If I don't have a passenger I can have a bigger drink because I use the drink holder on the passenger side of the center console.  Cupholders are a big decision when driving a Boxster.