I have their "single dump" kit and noticed a drop in a 1-2 mpg in the city. I was tempted to add a little to the pipe to help with the drone and maybe get the mpg back
With the one I bought, that comes out the side, I noticed that I was not having to give it as much gas as before. Mine comes out right behind the rear passenger tire at a slant. This could be the reason for the difference. Also, are you resisting the temptation to floor it? The sound is a good one, maybe this could be your culprit.