Are you using hub centric spacers? I see both hub centric and lug centric spacers listed on AM's website. They should be hub centric for late model Mustangs (haven't research when Ford switched to hub centric wheels). A lot of aftermarket wheels for 2011+ Mustangs probably aren't hub centric themselves. Switching to a lug centric setup isn't the end of the world, but thought I would mention it.
Also, there are notes on the product pages and in the reviews that stock wheels may not work. Looks like some people have made them work by cutting the lead in part of each factory stud off. That would not impact mounting strength, just make it a little hard to start the lug nuts if you every took off the spacers.
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Yea, I used hubcentric. Got them off Ebay for $114.99. To be honest, I'm not even sure the difference between hubcentric and lug centric, but figured I wanted hubcentric. There is zero vibration with them on at any speed.