If that is the case then I bet using a stub exhaust will definitely be an improvement over both open exhaust & silencer?

That s how we tested our engines. A stub of tubing the same dia as the original header and about 6 to 8" long (on a 125cc engine) would remove the undesired reversion of exhaust gas during the upstroke (& the resultant messing up of jetting/carburetion), and also improve exhaust velocity over open condition. This was the baseline condition for checking changes in intake duration, intake length and carb dimensions.

Anyone willing to try it out?