Zeng,

The Tower/GMS/Hobbico muffler will give you about 400-500 RPM more than an un-baffled OS 783 silencer, on larger props in the range; 11x6 for instance. On smaller 10x5-10x6, or 9x7-9x8, it could add as much as 1,000 RPM. It is a rudimentary tuned muffler.

Fuel consumption will increase accordingly.