The exhaust would be the better of the two upgrades in terms of performance, when i was getting mine i thought it would be about 400 also but it turned out to be 776. I guess it depends on what type of exhaust you get(True dual, y pype, h pype). The CAI would be the most economical upgrade and you can install this yourself then its done. As you said you can get one for as low as 80 or you can get a K&N that runs 260. I dont know why you would join an autocross club or advanced classes unless you plan on doing some extreme racing or drifting in your mustang lol its just like anyother car in terms of driving its got a stering wheel and peddles lol o by the way welcome stangs are the best!:bigthumbsup