For pure autocross fun and performance, I don't think you can go wrong with the RT-615.
However... Miatamoto and I seem to agree that the Hankook R-S2 is an outstanding tire for driving on the street, and it does great at autocross, too. Currently, pricing on the Falken seems to be better, and that's certainly something to consider as well.
Majik does well with the Kumho MX... Hopefully he'll chime in on his opinion of that one for street use.
OTOH... keep your eyes out for any holdover Falken RT-215, as that one is superior to its successor (IMHO) for autocross or street, although they tend to overheat during extended autocross outings.