If you were to spend, say, about 500$ on a performance mod (dyno tune, programmer, engine parts, etc...), where would you you invest the most for the best turn around (already have an exhaust system)?

Cold Air Intake and 2 better tires for the rear.

I just made an appointment for a dyno tune, 25 at the wheels w00t

I agree the Dyno is a great MOD BUT>>>>>>>What happens when you do more upgrades? -- More Dyno....

How much have you guys been spending for a dyno mod on a stock GTO?

Dyno tune is a waste of money on a basically stock car. If you have an auto car, pick up a new torque converter and you'll see dramatic gains in acceleration. Let me know your driving style and the car's normal use and I'll pick the stall for you. If you have a manual, save up more cash and put on some longtube headers. Then tuning it will really make a difference.