The flowmaster 50s are large mufflers. They look fine with dual 50s, but are visible from the rear of the car in the stock (and actually snug) position. Like I said, it looks badass with dual 50s, but I think it may look a little silly on a 1 side system.
As far as sound...they do have the typical flowmaster tinny sound (at least in my dual exhaust application) and do have the 'trademark' drone at 2200-2500 RPM, but not nearly as bad as 40s.
Idle sound is actually pretty badass sounding with a 260H cam. Most people (even at mustang shows) are very suprised to see a 200 in the bay after they hear me drive up. THey are quiet as can be at highway speeds. I actually can't hear the exhaust at all when cruising in 5th gear (not sure about other gearing options like 2.77, 3.03, or C4) with the top up or down. I hit 65 and the car just goes into quiet mode. Very easy on long distance trips. Again, that's with a T5 transmission.
Honestly, why not consider the stock V-8 muffler that places set for mustangs that is 1 in 2 out. It is easy to mount, hides away nicely, and still gives you dual out option and keeps it quiet.
Sound is always personal. Some people want loud, some don't. I like a deep throaty sound with a little bit of volume, but not much.
Slade