What mods should I start off with?

Arikara, keep in mind I'm fairly new here and haven't done any performance car building in 20-25 years, so don't really know much about these new fangled computerized cars. Check out my thread from last night. I just swapped the rearend gears in my 08 GT and its made a world of difference. I'm thinking my next upgrade may be the AL driveshaft also. Swapout should be fairly straightforward, though I imagine a torque wrench may be needed to torque the bolts to a specified #, I haven't researched the driveshaft fully yet. I've got the K&N CAI on mine and it doesn't require a tune, though I've got a tuner on order from Brenspeed which should boost HP roughly another 20. Adding the CAI did make a small but noticable difference. According to FRPP the gears aren't covered by warranty, so I don't know if its worth what the dealer will likely charge you to do the swap. My dealer wanted $800 for just the labor. Find a trusted speed shop in your area and see what they'll charge. If you develop a problem with the gears that's related to installation a good speed shop should make it right. I'd already ordered the gears and shim/sleeve kit from Ford, so installation only for me was $375 + the rearend oil at a local Mustang specialist speed shop. The Brenspeed tune and an AL driveshaft should net me roughly another 30 HP and probably somewhere close to that in torque, which will make my rear wheel HP and TQ close to what Ford rates the motor at.