Yeah, an off-road midpipe & the stock catback can make for a somewhat obnoxious exhaust combo IMHO.
There's a couple of options here.
First, you could choose to add cats to the O/R pipe (weld in a couple of "Carsound" cats - parent company of Magnaflow, they make excellent catalytic converters). Doing this will probably provide the biggest decrease in sound volume. Today's cats flow very well, so don't fear the old wives' tale associated with cats robbing you of horsepower - it's simply not true with the technology out there today.
If you're still not satisifed with how loud it sounds after you get the cats welded in, you could weld in a pair of straight-through mufflers or add an entire catback (ie - Magnaflow, Bassani, Dynomax, etc) to help "smooth out" the exhaust note. These mufflers tend to have a much more mellow sound than their chambered counterparts (such as Flowmaster, MAC).
My combo (see sig) is rather tame & on the quiet side, but once you mash the go-pedal and are WOT, the exhaust note comes to life - so people will hear you. But for everyday driving, it's great. I'd describe it as "stealthy."
Finally, check out
www.mustangexhaust.com if you haven't already done so. There's a plethora of mustang exhaust information at this URL, research away. Also, do some "in person" listening tests/demos with other 'stangers in your area. Hearing actual exhaust combinations is the best way to go about it, because some audio or video clips don't always allow for a "true" sample of the way something sounds.
