Yeah it would add some HP. Anything is better than stock, especially if you are going to turn a single exhaust into a dual setup. o/r pipes open it up for a little more than a catted pipe.
it will be louder but wont help THAT much on power. It will feel a little peppier though. All stock Mustangs are too damn quiet lol. My v6 with Bassani was louder than my stock GT. Not for long though, I had that thing opened up within a week of purchasing it. I cant stand quiet Mustangs! lol