I copied some good muffler information from the November issue of 5.0 Mustans & Super Fords. For performance and sound comparison of several mufflers, look below.
These will allow you to compare low end torque, and high RPM HP, and sound levels at differant RPM levels.
The test vehicle was a 1990 Mustang with a 5.0, built as a 306 with Twisted Wedge heads, Thick Flow Track heat manifold, 75mm throttle body, AFM N-91 cam.
The Borla XS was the best performer, but at over $150 each, out of the reach of most. Next on the top of both low end Torque AND High end HP, is the Magnaflows, and with a choice of coated steel, or stainless.