I did a test run today with the engine using the muffler baffle insert and it definitely lowered the noise level quite a lot. I was surprised it worked that well too. It also only caused the engine to lose about 200 RPMs off the top end with a 13x6 prop. Plus the idle speed and idle performance improved a lot too. I could idle the engine lower than before. So much so that the engine starts to develop a slow surge with it changing idle speeds going slower and faster and slower again. So it turned out to be a pretty good success story for the engine.
Here is the engine going wide open and it didn't lose a lot of power with the baffle insert.
Showing the slow surge or idle oscillation it was doing. It was slow enough to take pics of it.
Here is a short video clip of me running the engine with the baffle inserted into the muffler. Also at the engine of the clip I alternate with the baffle out, then in, then out and back in again too. It does make a big difference in noise levels having the baffle inserted in the muffler.