I think pipes are designed for the time period (degrees) that the exhaust port is open primarily and how often that happens in a given time. A shorter stroke usually makes some bettter RPM but there are other things involved. A shorter stroke usually means less torque and in a small engine you need a certain amount of torque to get you up to high enough RPM to get the pipe working. This would be less of an issue with a mid range pipe but then mid range pipes are not going operate the higher RPMs that the shorter stroke is good for. In other words it's all about balance of torque and horspower. Transmissions can keep you in the higher RPMs and larger displacement kits can improve torque. This is just a summary of a few concepts and not the whole enchilada