well if your street tires are heading down the final stretch I would not get the r-comps anytime soon. Race what you got and make your mistakes on the old set.
Once the courses start to look repetitive to you invest in new tires. You don't want to make all your mistakes on new tires because then your stuck with chewed up tires for the rest of the season. Those Rcomps, particularly in hot summer, are easily shreded on a poor surface.
Do a search on the Rennlist Autocross/competition forum for optimal Boxster sizing.
http://forums.rennlist.com/rennforums/forumdisplay.php?f=85
and if you haven't already, you should join your local PCA. Here in Jersey one of the big dealers give a 10% discount to PCA members which comes in real handy when you do your pre-autox/track annual service/alignment/brake flush/oil change/etc.