I bought my BBK o/r x for 200.00. It was too loud with the Flowmasters, so I dropped them and put on some chambered Turbo muffs. Still too loud and the smell without cats reaked.
Here's what I did... Bought 2 Catco high flow cats from Summit Racing .com, of course. And had the o/r x-pipe cut to pop them in (35 bucks at Midas) and I'm running a quiet BBK x-pipe WITH cats and muffs. It's fairly quiet, which I like. I can listen to the motor now and still get a little thunder from the exhaust, without sounding obnoxious.
I like my stang to have a classy smooth sound, but still kick ass when the blower goes to work.