I'd buy a turbo back exhaust system if i were you though

 

My vote here is that you guys were harder on the 'go' pedal than normal. It is just normal to want to hear that new found sound!! If you can stop listening for it and drive like you did before, your fuel mileage should return. If it doesn't, then I would look for something wrong. Oh, and a 5.9L doesn't have any way of 'knowing' if the exhaust pipe is there or not. So it wasn't the computer messing with your mileage either. 'Straightening' up the exhaust flow will help with mileage; but it is a small amount. Most of us won't even notice the small .1 or .2 mpg improvement. Hopefully you will see a drop in EGTs though.
I don't think I ever posted pics of my 'selectable' muffler, did I? I might have to do that. . .

 

I want to see pics of the selectable muffler.

 

i just did mine with no probs added a triple dog a few days later and lost about 1mpg but iv been leanin on her a little bit

 

I have the stock piping with a muffler delete and the cat is gutted but still in place, so basically straight piped...it's damn loud, especially when it's floored...didn't notice a change in mpg's....but really noticed the looks from people unfortunate enough to be on my exhaust side....windows go up quick...

 

Its probably due to the face that your truck now sounds badass and you give it more throttle due to the sweet sound of the all mighty Cummins.