I suspect that the OEM muffler on the Evo 10cc engine is more of a "tuned" muffler design. So the muffler has less of a performance loss when used with the engine, sometimes even a gain. The OEM muffler looks more like the old Performance Specialties Ultrathrust mufflers (aka Nelson Mufflers) or the Jett mufflers. Thus if you use different mufflers it could affect performance.