Get the tuner 1st. My experience is with the SCT XcalII. It will wake up your car and with your AT you can firm up the shifts like you were talking about. Do a search for sct or tuner or handheld or programmer or whatever. There is a lot of info about it here ;o)
Do this 1st and see how you like it. If you are still wanting more gears would be good, 3.73's or 4.10's by FRPP I hear are the way to go.
Osiris - '07 Shelby GT500 - Black