I love the stock exhaust and wouldn't change it for anything. On my blown '86 GT I had several different aftermarket exhausts (Flowmaster, Bassani, Dynomax, Mac), and all of them had too much drone and were raspy inside of the car. Outside the car they sounded incredible but I never got to hear the good sounds unless I propped the hatch open with a box--which I actually did a number of times.
For me, the stock exhaust is perfect--you hear it when you're on it, and it's fairly quiet for cruising. Also, unlike the Fox bodies, the stock system is 2.5" diameter and mandrel bent, so you don't really need to change it for performance reasons either.
Owner1