Congrats on the new GT! Ive had mine since Feb. and have loved every minute of it.
My first mods:
Roush axle back, had it before I had the car, installed them the night I got the GT. Absolutely love the sound and get compliments all the time. They are loud, but not obnoxious and zero drone. I could easily see how they would be way to loud with a cat delete though.
Lowering Springs, also had them before I got the car home. Installed them and the adj. pan hard bar that weekend. I got the SR Perf. springs, 1.5" drop, ride great even with the stock stuts and shock, but wish i had gone lower. Im looking at sportlines or H&R supersports now, most likely with a shock\strut upgrade now as well.
Ive done some other cosmetic stuff but those were the major mods so far.