Usually backpressure might help with idling, but not much else. Now sometimes there can be some subtlities where a engine needs just a little bit of backpressure to run better where it might be more tractable with a little backpressure.

The glow engines use muffler pressure or back pressure to pressurize the fuel tank so the engine can draw fuel more easily. But with gas engines, if you are using a fuel pump/regulator system, then you don't need muffler pressure of course.

Yeah the muffler could leak a little here and there, it depends on the person as to whether it is too much or not.