CAI- K&N filters are great, but you're not adding all that much power unless you remedy how the throttle body is bringing the air in. In my T/A, there was the plastic air box, to the MAF, to a ribbed plastic elbow which was real goofy, then it went to the TB. I got a K&N CAI, which pulls the air from outside of the engine bay thus being cooler. I also deleted the elbow for a Trick Flow ceramic elbow. The elbow looks nice but it was overpriced. The cold air intake was expensive but certainly worth it.

Exhaust- You won't feel anything here unless you open that exhaust up. Go long tubes or go home. You will feel a big increase in power right off the bat. I always delete cats too but there are no emissions here. Catbacks usually flow better than stock and they all sound slightly different. I'm a sucker for Magnaflow, Pypes, and GMMG. Don't even think about putting a Flowmaster on your Mustang. Pypes has some good kits you might like.

With every vehicle I am sure there are "free mods" to help boost a couple ponies just by doing something simple. On my car it's the throttle body bypass. This eliminates coolant going through the TB and warming up the air that travels through.

Hit the forums and look into these. Good luck.