A lot of the reason no one really tells you what the gains would be on a cat-back (or any other mods) is because no one really knows, all cars are different, and respond differently to mods. On top of that, some people read their hp at the crank, and some prefer to know what they are putting down at the rear wheels. They can (and usually do) give a ballpark number, sometimes it is inflated a bit, but it is a ballpark nonetheless.
Given that you have a 2010 GT, I would say your best bang for the buck would be a CAI/Tune combo from American Muscle, and a catback exhaust, if you have some extra cash, you can go for long tube headers as well. You are worried about the tune voiding your warranty, don't. The American muscle tune comes on a hand held tuner, and it takes about 5 minutes to flash your car back to the stock tune, and the dealer will never know (chances are they wouldn't know if you DIDN'T put the stock tune back on before you went in for work, unless they needed to check the ECU for something). Tunes REALLY wake those 4.6's up. Some have gained as much as 20-30 WHP.
The reason I suggested the American Muscle tune though, is because they are a top notch company, when you purchase a tune from them, you get free retunes for LIFE. So, you order the tune and all you have on your car is a CAI, they write the tunes for a car with a CAI. Say a few months later, you get Longtube headers, Highflow cats, and Magnaflow exhaust, you shoot them and email listing your new mods, and they will email you updated tunes, that incorporate the new mods, they even tune for forced induction! As far as I know, no one else offers this kind of service. Also, the SCT tuner they use, can store 3 tunes, so you get 3 different tunes. You chose the octane rating, and street, performance, or race, for each tune.
I don't work for American Muscle, they just do great things for the Mustang community, I'm sure most Mustang guys would agree.
