Like Dean said... seat time is #1!!
Also, like Dean said... the STi is *very* capable out of the box... You've seen my car and all the crazy work I've doe to it... and to be honest, it's just about the equivalent to a stock STi: I have a little bit better handling due to the coilovers, but a stock STi will kill me on power.
In fact, right now MattR (who's racing a Street Tire A-Stock STi) and I are *tied* for 10th in the Open PAX standings.