No, you don't have to modify an engine to see real world gains above the advertised red line. The FOX .36 Combat Special is an excellent case in point. They were rated for 18,000 rpm......only losers ran them this way, most competitors ran them above 20,000 rpm, no mods were needed, just drop down in prop size and fly models capable of exploiting the bonus rpm. If you fly nothing but lead sleds, you will never realize the gains..