There's no denying these 11-14 GT's are incredible bone stock. It's not surprising to me that you're already impressed with the car (I am too!).
It's clear you're happy with the cars power output and don't want to void the warranty. There's nothing wrong with that either. As far as suspension goes, there are a few upgrades that will show significant improvements without voiding the drivetrain warranty. One of those upgrades would be rear end lower control arms
. The factory units work for daily driving, but during hard acceleration they will twist/flex causing the car to lose traction. Any quality aftermarket set of LCA's will significantly improve strength here and prevent loss of traction.
My next suggestion would be lowering springs
and an adjustable panhard bar
. Lowering springs will not only give your Mustang a more aggressive stance or ride height, but they'll drastically improve the handling or body roll. By lowering your Mustang roughly 1.5" all the way around, you'll see these improvements without sacrificing ride quality. The adjustable panhard bar is needed necessity when lowering your Mustang's ride height to retain the centered rear housing.
Other than these two or three suspension upgrades, I suggest just enjoying the cars performance for a while longer. See if there's anything else you have distaste for down the road and then go from there. Most importantly, enjoy your new ride!
Of course, if you do need any assistance with parts or questions, you can always feel free to contact me via PM. I'm here to help!