I stayed home sick the last two days and I got pretty bored and I ended up watching random YouTube crap. I stumbled upon this guy who does a “how to” on installing a mid pipe and cat-back on his mustang. He also did a before and after dyno. The video is kind of long, but he gained 10 rear horsepower. https://m.youtube.com/watch?v=v-s4ZCsC1oY
I didn't have mine dyno tested when I done my exhaust setup, but my exhaust went on the same time my new engine went in, and of course it was a lot more powerful. A mid pipe on the 4.6 picks up a good bit a power, but it's really from 2,800 rpm and above. On average I've seen a full exhaust with long tubes are 25-30 rwhp which isn't bad considering that's rwhp, so your looking at around 35-40 flywheel. What i would like to see is a test on different chambered mufflers, now that would be cool. But as for my x pipe, I guarantee it picked up 10rwhp, and that goes for any stang.