remember, this is just my input.
There is a fact that Flowmasters are superb and high quality, so on the performance side you won't be disappointed. however, the issue of sound seems to be the big sticking point, different sounds are better to some ears than others.
some examples, muffler placement in the system (forward, toward the rear more), one or two mufflers of course sounds different, the series, your cats. (if they're there) etc.
I have heard from some, and been discounted by others (owners of all types of vehicles not just Expy's) that some Flowmaster mufflers have a "drone" while cruising at speed, some like it, some don't, some ears don't pick up on it, some don't have drone at all.
Personally I have a Magnaflow single cat-back 2.5 inches inlet, 3 inches outlet, basically O.E. upgrade replacement. At startup it is louder than stock, but not wake the neighbor or the wife at 4 a.m. louder.
At idle to my ears is only just slightly throaty and pretty much as quiet as stock.
Under pedal at about 2k-2,500 rpm it sounds best; which I describe as a mellow growl, low tone/pitch rumbles similar to the GT mustangs just half as deep/throaty/open/loud.
I love it.
Perfect if you drive on the Highway a lot (avoid the drone) like I do, so I get to hear it every time I pass some dude in a Prius ((in that 2000 rpm range)) So I imagine dual exhaust must sound dual as good right? ha
so talk to a couple of the guys at a couple different muffler shops, take some notes, if you notice a pattern emerging.....
My only other thought is I wonder how super good a dual Borla exhaust would sound?
I know I'd have to piece it as hey don't make a kit, but holy cow comes to mind.
as I finish I found this video on youtube, He doesn't actually start the truck until like 1:07
http://www.youtube.com/watch?v=pNwatx9iXuo