Does the Rotax 'After-Muffler' part 878967 really work, reducing noise?.
I'm running a Rotax 503DC on my Gyro and constantly operating between 6000 &
6600 rpm.

The after muffler and intake silencer work very well. You will still have the
major source of noise to deal with, the prop.

Be prepared to have to re-jet the carbs.
Tom