Ford Racing won't void the warranty if that matters to you.
No cat-back voids the warranty according to the ford techs. As long as you don't mess with the cats, you're fine as far as warranty goes for exhaust.
as far as suggestions for the OP: Personally, I think Ford Racing and Corsa have the best sounding systems out there. And they're great quality. A lot of people also like Stainless works systems.
I have ford racing Sport system on my 15' and It sounds GREAT! and had perfect fitment/easy install. I like the Corsa sound a little better, but it wasn't available when I bought my system. I might switch eventually. If you want a good sound reference you can check out the youtube video I made https://www.youtube.com/watch?v=H-_YJqVQSAw
Hope this helps!
2015 mustang GT, Performance Pack, Recaro seats, All-weather floor mats.
VIN received: 10/16/14