AP makes a huge difference on the WRX automatics. Before, I could not stand driving my g/fs car. My 98 RS was faster and more responsive (still more responsive though). Before the WRX could not power out of corners, would lag terribly, and generally make you feel like you got screwed. This all mainly due to Subaru's inability to program a trans ECU.
After we put on a downpipe/uppipe and flash stage 2. Way better, responsive and fast. Can power out of the corners, and makes performance driving alot more fun. My RS is still faster on corners though (500lb weight advantage). Even if the WRX has several thousand dollars worth of full suspension/tires/wheels/brembos.