Purchased mine for Gremlin Combat. Compared it to an OS25FX and with the Stock Weston Muffler there was NOTHING between them both on 15% and APC 9x4. RPM's were within a couple hundred of each other.
Problem appears to be that the weston 25 stock muffler chokes the motor. In fact the passage from the engine into the muffler body is restricted so much by a single drilled hole that I contacted Weston in the UK to make sure it was actually correct. The OS motors have no such restriction for gasses getting into the muffler body. I still think the weston may show promise, but only if your rules allow non-stock mufflers to be used.