at idle, dont expect it to be much louder, but when you get on it, its gonna scream like the devil. get some Flowmaster 40 series mufflers welded into your stock catback and you'll have one sick sounding stang.
I have the mac lt's and catted h with a magnaflow catback, and it is real nice at start up, one neighbor always comments that he knows when I start my car and the neighbor up the street just recently commented that my car sounded awesome and that they could always tell when I drove by. Not too loud cruzing and very powerful sound at wot !!!