its called "high idle" meant to warm up the car quicker by idling at a higher speed, this will cause less engine wear as during idle the engine is not under load.
a muffler might give you .1% of difference on our motors, on a high performance engine mufflers may make a difference but its the pipe size that matters.
Thirdly, your car WOULD NOT idle at 300rpm, that engine speed is insufficient to maintain the stroke cycle.