A muffler restricts the exhaust and will lead to decreased power from the engine, even for properly tuned engines. Removing the muffler will remove the restriction in the exhaust and undo the power decrease, and also increase the noise.
I can attest to that, my old Saab performed terribly (performance and fuel economy) during the time I was running around with a pierced and corroded exhaust mid-section flexi-joint.
