I have a 2012 GT
with the Roush axle back exhaust ... I want louder but can't go cat-less since I need them to pass inspection.
I was considering the Ford Racing "Boss" quad setup. It's pricey ... but it's side pipes, and side pipes are cool.
I watched some Youtube videos of Boss's with aftermarket axle backs ... they sounded good. Louder than mine, but not horribly so.