If you're referring to the Dynomax Super Turbo cat-back, I've had it ... and, technically, I'm still sort of running it, too. I kept the flow tubes and tailpipes from the cat-back when I got my Pypes Violators and just swapped out the muffs (needed to trim the tailpipes about an inch on both sides, because the older Violators were a tad longer in length).
Frankenstang also has a Dynomax Super Turbo cat-back on her '86 GT 'vert.
In any case, they're about as quiet as you can really get, and as close to stock as you can find without really choking things off. Given, they don't flow nearly as well as any straight-through muffler will, but they definitely are quieter than most, all-around.
HOWEVER ... it also kinda depends on what sort of circumstances you're wanting quietness under. That is to say, there are some that are even quieter than the Super Turbos at idle and easy driving, but they will open up and roar at WOT; a couple of examples would be the Dynomax Ultra-Flows and Hooker Maximum Flows. I've had the latter of those two and loved those almost as much as my Violators - I've ran them on a '94 Crown Victoria Police Interceptor, a '76 Gran Torino, and a '91 Trans Am - however they were a bit too tame for what I wanted out of my notch, and the price of the Violators (at the time) was impossible to pass up.
If you search a bit, you might be able to find that one thread where someone copy-pasted an article from a magazine that did a decibel test on the various kinds of popular mufflers for Mustangs, and gave a comparison of them at idle, cruise, and WOT. It was kind of unusual that some of the straight-through types could be the quietest on the list at idle and cruise, and yet almost peg the decibel meter at WOT, whereas a lot of the chambered mufflers had a less dynamic difference in sound between those three instances.
IIRC, the Hooker Maximum Flow was about the quietest on the entire list at idle, still among the quietest at cruise, but also among the loudest at WOT. If you're looking for something that's stealthy until you really romp on it, especially a muffler that's not going to choke off your exhaust at all when you're running forced induction, then I'd recommend those as the best way to go. I don't know if you can really get Hooker Maximum Flows in a pre-set cat-back package, but you can easily buy a pair of flow tubes (or are you running a single 3" or larger exhaust?) and some stainless tails from either Dynomax or chromes from MAC, and then buy the Hooker muffs (or whichever brand you prefer) to throw inbetween 'em - a few bucks extra, but not a HUGE added cost, really.
Flow tubes = approx. $50
Tailpipes = $75 to $100
Mufflers = approx. $65 apiece
However, I'll still stick by my endorsement of Pypes Violators as an all-around kickass muffler. :nice
