Ok if SCCA National Solo is your game, here are the results for 2010:
http://www.scca.org/eventresults.aspx?event=15171&hub=6
*Cay S/Box S ruled the roost in A stock and destroyed the Vettes
*Honda S2000 owned B stock
*C stock was a mixed bag between Mazda Mx-5, Porsche 986 Base Boxster, Nissan 370
So yes, a base Boxster is very competitive at the highest levels of SCCA Solo AX under 2010 rules.