245 is awfully skinny for AX. I'd run 275 square. However my favorite setup on Boxster was 315/30/18 and 295/30/18 a6s.
If you're not ready for DOT R tires Hankook RS3 is awesome ax street tire and they make some good sizes for Boxster with 17s too. NT01 is nice treaded DOT r tire that will be much more forgiving to your wallet.
They are on sale at Performancetread.com now too.
Where do you plan to autocross? Do you run with RSCC ? Be carful about compounds you choose if your running at cold venues. road race compounds like Hoosier r6s need heat to work well in ax. I'd say two best dedicated autocross tires are the Good Year Eagle RS DOTR in AUtocross compound and the Hoosier A6 DOT R. In cold weather the good year actually works better!