I haven't had it dyno'd, but my by the seat of my pants testing definitely tells me it has more hp. When I get on it, she leaves everyone else behind.
As far as mpg, before everything, I was getting around 12 to 12.5 around town. Now I am getting around 14.5 to 15mpg and for some reason I am driving it a little more aggressive lately.
The correct MagnaFlow Y-Pipe for our truck is #10778. It has 2.5in. Duals/3in. Single ends.
Hope this helps. Let me know if you have any other questions. I am not an expert like some of the others on here. But I try to help when I can.