I was wondering what was better for gas mileage and hp increases. Would it be a cold air intake or a exhaust system with high flow cats?

i think they will both make about the same amount of differnce. you may not be able to tell the difference from just one but with both mods you will notice some gain in hp and mpg.

If you can't get the waste out of the cylinder it does not matter how much air/fuel you try to stuff into the cylinder. Don't put the horse before the cart. Size to a man is one issue but the efficiency/flow is the true measure in your case when looking for an exhaust. There are some awesome exhausts out there that sound great in the bottom end and yet at cruising speed they canacel the harmonics out so when you are cruising you don't get the constant droning sound that drives you nuts on the highway.

Good Luck