Sounds as if you're classed for STS2 (or whatever it's been renamed. STS? ST?) Anyway the Miata may not have street cred but it's a nearly perfect autocross car. I just got done racing today...I finished second in our PAXed pro class; I was in a CTS-V in ESP and I lost to an ES Miata. I beat him by about .4 on raw time but he got me by .6 or so on PAX. That's another thing: in a street tire class, PAX will be your friend. For advice I'd say buy the best tires legal in your class. Ask what the fast guys are using for tires, but I think right now the Kumho XS and Bridgestone RE11 have a lot of fans. Tire alignment will be a big deal too. Race as much as you can; seat time will make you faster than any part you bolt on the car. Remember slow in, fast out. The Miata is a momentum car so if you get out of shape and off the gas other cars will hand you your ass. Ask to ride with the fast guys. Watch their hands and the line they take. Listen to the engine to determine their footwork. Walk the course as much as possible before an event, and walk it with the fast guys.
ETA learn to look far ahead. Don't just stare at the next cone in front of you. You should be looking through the side windows just as much as you look through the windshield.