I posted this in the other thread too, but I'd say lighter wheels and better tires. I'm running the 16" Konig Dekagrams wrapped in 205/50 Indy 500s and it's a great combination. The additional sidewall significantly improved the ride quality, and the lower unsprung weight from the wheels (14.5 lbs each vs 22.5 lbs each stock) makes the car feel more nimble.
If you're dead set on getting an AP first, get a better intercooler at the same time. Depo Racing and Whoosh are the best value from my research, but whatever custom tune you get with the AP will benefit more with a better one.
From everything I've read, intakes and exhausts only make more noise, not hp/tq, if you're still running the stock turbo.