If you want to add some performance right away and relatively cheap without having to chase down a half a dozen people with different exhausts, go the CAI route. There was also a spreadsheet done recently with cold, hard performance data which takes a lot of the subjective part of deciding out of the equation, something you can't do nearly as easily with exhausts.
Or if you don't mind spending twice as much (NOT including labor) and voiding your warranty immediately, you could go get the 3.73 gears. Costly, but probably worth making a few 2011 Mustang V6 drivers wonder if they got lemons seeing a V6 Camaro keep pace with them.
