Just this weekend I actually installed Flowmaster 40's into my car, and I would have to say that the sound on them are amazing. But allow me to layout how I drive in my car first.
I drive with the music very loud, but when I have it low I can hear the Flowmaster making a sweet noise. I wish it got louder when I floored it, but since I have a S/C, the whine from the S/C kinda overpowers the muffler.
Performance increase??? I would have to say that I fell NO performance increase whatsoever. But let it be known that I didn't get Flowmasters with the intent of performance increase, I was looking 1st for the sound and 2nd for the slight MPG that I would get over the expanse of owning this car. If I got a simple 1/2 better MPG, I'd be happy (every 30 refills I get a free refill which is 40 bucks, so after 90 refills the flowmaster pays itself off).
Where did I get my Flowmasters? Off of Ebay, get them for 40 bucks, and the part number is 42441.
Install - Local exhaust shop for 60 bucks or less