Mine is stock, so I can't offer real world advice, but my guess comes purely from logic. Honda is under great pressure from the government to maximize fuel economy in all of its vehicles (CAFE standards). Logically, the stock muffler is probably meant to do just that, while minimizing exhaust noise. If your priority is fuel economy, then stay stock. If you want a cooler sound, then change. My philosophy has always been that the Honda engineers know a helluva lot more about Honda engineering than I do, so all changes that I do are purely cosmetic. That's my two cents worth.