your best bet for a mod to increase acceleration off the line, and also stay under $500 is simple.
uppipe: ~$200 (install another $200 unless you do it yourself). decreases turbo lag by about 1K rpms, adds hp. boost earlier is better.

there's tons of info on this all over the board on how to install yourself, and how to prevent leaks.
manual boost controller: $60 even if you set the BC at the stock level of 14.5 psi in third gear, you will now achieve full boost in first and second gear. the factory BC limits you to ~9psi. you'll fly through first and second gear in a hearbeat. increasing boost beyond that will yield more power throughout the power band. some say ~10hp & torque per pound of boost added. stay at or below 16.5 if you want the car to last.
K&N panel filter.: $50 more airflow to compliment faster spool up and higher boost. 5hp increase on the box, but with the previous mods, should help to make even a little more.
so you blew budget there by about $10.
dR