if you read any of the SCCA forums or GRM everyone is going to tell you RS3's. depending on the size you are looking for the most common ones are on backorder. 255/40/17 is backordered for like 2 months. i just ordered a set of 225/45/17's for my car from tirerack and they should be in on monday. they are like $120 each and the fastest street tire out there right now.
here are the results from this years GRM street tire tests. i am not sure how many miles you are planning to get out of the tire but RS3's are the best bang for the buck. just about everyone in ST classes is running them, and hankook doesn't have a contingency sponsorship plan (that i am aware of) like other manufacturers if that tells you anything.
they are up to a full second faster than the star specs that everyone was clamoring over last year.