You're in LA, right? I'm about 2hrs east of you.I have Flowmaster 40s, they sound great, like an actual muscle car. I hate the glug-glug-glug water sound that I hear from many newer Mustangs with aftermarket cat-back systems.
But mufflers are only half the battle. The first thing I did in terms of exhaust was switch to JBA shorty headers (California compliance, not a lot of choice there). The sound was night and day after the switch, even though the rest of the exhaust was the garbage stock stuff. I've read that some on here switch to full-length headers and/or no cats/X-pipe or whatever, which is fine if your home state doesn't care. I'm just saying that the sound can change a lot with even shorty headers and stock mufflers/Y-pipe.
My view is if your plans include headers, do those first, then figure out how much more "sound" you want out of aftermarket mufflers because settling on mufflers first and thinking you nailed the sound might not last once you swap out the exhaust manifolds for headers.
Anyway, my plan (later down the line) is to just get a set of OEM exhaust manifolds and have them ported instead of getting a set of shorties. My understanding from reading various threads here is that ported OEM manifolds give you better HP than shorty headers. Being in California and all, I'd like my engine to be as "sleeper" as possible. Having ported out OEM manifolds will achieve that objective while also gaining more HP .
As for me, I replaced my two primary CATs a few years ago after one of them went bad with Magnaflow High Flow units (CA legal, of course). Even with that little repair / upgrade / mod and the rest of the exhaust system being OEM, it gave it a little bit of "glug glug glug" tone at lower RPMs. I can't wait for the day that I get a full catback system in place. Oh, and I'll be doing an X-Pipe with a set of secondary CATs immediately behind the CAT as my smog guy said that's perfectly CA legal if replacing the stock Y-CAT-pipe.