I agree on the CAI, and if you still have the factory exhaust you could get a different mid pipe and catback. Most people want to make their stang sound louder and better, so this would kill two birds with one stone.
Welcome to the forum. CAI would be down a little further on my list. Exhaust, gears, high flow filter, udp's, aluminum d/s, subframe connectors, tune, control arms and d/r's. Of course you could do what I did first, put a line loc on it. It is a really fun mod, but is really hard on the tires