Help With My 2003 V6 Mustang - Get More HP

TB will do nothing unless you have forced induction. CAI might make 5 or 10hp. Chip will give you 20 or so.

For a power increase you can feel, you can either spray it (for the track), or for everyday use have a look at Super Six Motorsports' cam and intake kits to pick up 75-80 hp. The installation is a lot of work, but probably still less than a turbo.

I agree, cam and intake would give a good amount of power, and if your looking to do exhaust put on a dual exhaust system, not just one muffler. Systems are not that expensive, and a true dual exhaust could very well give you another 10 hp. The only disadvantage with that is you might lose some low end torque depending on the size piping you use.