I have to agree. No real gains and even possible losses.
Most rigs now days require the "scavenging effect" to assist in getting the exhaust out. With to little resistance, the exhaust gas velocity can drop to the point of eliminating the benefits of the scavenging effect. By doing what you are thinking about, you may reduce the scavenging effect of the exhaust pulses, actually causing a negative effect on your HP and mileage.
This is the reason a true dual exhaust(or running with no muffler) is not recommended on these small/high RPM engines.
The "scavenging effect" is when the previous exhaust pulse helps to suck out the next exhaust pulse.
If you were to run WOT all the time, the "scavenging effect" would not be an issue(as in racing). If you are racing then your mileage doesn't matter much anyway.
