The more agressively you corner, the more the tire on the outside (with the weight) rolls onto its *outside* sidewall and wears the outside of the tire.
To counter this, you dial some negative camber into the vehicle which means that if viewed from the front of the car, the tops of the tires are closer together than the bottoms. The front tires are leaning inward. This way, when you corner the tire rolls over into a flat position. The wider footprint give you more grip and it doesn't chew front tires up. However. There is no free lunch. When you are driving down the road nice and straight, the vehicle is riding on the inner shoulders of the tread.
So. They aligned your car. Yay. Problem is (and keep in mind I've been the dealership tech you took it to for thousands of alignments) they don't want to move caster or camber unless they have to. If you're within the arbitrary "green zone" on the computer, and both sides are closely matched? He's going to leave it alone, set your toe, and print out a pretty picture for you to take home. Remember, he gets paid an hour or two to do an alignment in most cases and it can get very time consuming once you start adjusting major suspension components. Especially when, like the Mustang, they don't *have* camber adjusting bolts installed from the factory. I'm pretty sure that's the case.
You will probably, as a gut estimation, want to bring it to a shop which will take about 0.5 degrees of negative camber out of your current setup, readjust the toe, and send you on your way.
(The guys who have caster/camber plates often don't realize that adjustments to camber affect their toe, because tilting a tire inboard or outboard moves it closer/farther from the steering rack, and the tie rod end is a fixed length, so the tire toes in or out a bit when you adjust camber.)
2014 Sterling Gray GT Premium with silver stripes / MT-82 / Brembo / Recaro
Steeda Sports | Koni Yellows | BMR Panhard and LCA Brackets
MGW shifter | Borla S-types