I decide to try the Dr D exhaust to see how it compares to the stock set up with the 5/8" header mod. The Dr D exhaust is lighter, and the fit is perfect. The set header design uses the same OD for the header it self, but without any type of restrictor, unlike stock. The mid pipe is about .2" bigger in OD, than the header. Performance wise, low end is good, although a tad less than the stock set up with the 5/8" mod. Mid range is comparable or may be a bit better than stock. The high end is better than I expected, with quicker acceleration at WOT. The only real down side is the noise level, at or near full throttle. As expected, the Dr D is louder that the stock pipe with the header mod. But so far we only did a brief test in front of our house, we won't be able to ride until next weekend. I don't know if they make a quite core for the play bikes, but if they do that may be an option. The bike may also sound different at Gorman, at higher altitude and more open spaces. Temp wise it was warm out and that may have had an effect on noise as well. The only change required was to open the air mix screw a .25 turns out. So it's easy to change out the pipe. Cost wise it's not the first mod I would do, but may be something after you have completed other suspension and performance tweaks. My son doesn't race, so more top end isn't the most important thing, but it's a nice to have.