What To Add To Improve Performance

My Mustang is a 2009 GT with auto trans. I put a new axle back exhaust on to improve on the stock sound, but there are no other mechanical mods. I've done a few exterior mods like gas tank Bullitt lid, chrome on taillights, etc. I have driven it about 28,000 miles since buying it with 12,000 miles August 2010, so now I have 40,100 miles. What suggestions do you have for me to make it a little quicker and stronger. I am a 70 + guy without lots to spend, but willing to look at options of which I assume cold air might be the first. Any suggestions would be helpful. Thanks very much.
  • Sponsors (?)

Underdrive pulleys and axle back exhaust can be done without a tune. Not a huge gain, but it helps. Pulleys help more, but the axle back sounds good. Pretty much everything else also requires a tuner. What Jimmy says is probably the best bang for the buck if you can spend that much.
SCT X4 tuner and gears 3.55:1 or better.