It's not the tire size, 235/55/18 is a factory GT tire size, and when I had my 2011 GT, I ran the factory Pirelli A/S 235/55/18's up to 150mph many many times.
If you're going to be driving at those speeds at the drop of a hat (unplanned in other words) than you should be checking your tire pressures regularly.
Since the tires are new, I have to ask, how many miles are on them now? I've always noticed brand new tires feel squirmy at higher speeds until the rubber release agent wears off completely.
Also, the tires you bought are an all-season high-mileage rubber compound according to the Yokohama website, meaning they're not very sticky, so regardless of the speed rating, I would be cautious with that specific Yoko tire model period.
With the ABS feeling you encountered, it could have been just that if the Dynamic Stability Control was kicking in trying to save your butt.
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2015 Mustang GT - Black w/Perf Package
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