As Frank correctly points out, this topic has been discussed many, many times. There is even a sticky on the topic with audio clips dating back to Rally I when we made some audio signatures.
Sound of our GT is a very personal and subjective topic. Ralph's long tube headers sound good if you like LOUD. At Texas World Speedway there was no mistaking when Ralph sped by. Too loud for my street use, but that is my opinion.
There are many makers of aftermarket exhaust systems for our cars. Come to a rally and hear each of your candidates in person is what I would suggest. I am very happy with my Borla, but that is just me.
Also I would be very leery about horsepower and torque benefit claims, unless possibly a cat-delete. The OE exhaust system as delivered might not "look" high performance, but it is a very good, free flowing system allowing very little (if any) performance gain. Credit the Ford design team (again).