The combat guys generally acknowledge that their (F2D) mufflers are more megaphones than mufflers, and make little, if any, reduction in dB's. It's driven by the rules. The rules mandated mufflers, but apparently didn't put any dB limits on them, unlike F2B, which has stringent dB limits. The folks in Ukraine and Russia (where the engines are made) don't care about anything but horsepower and meeting the requirements of the rules.
Note that the AMA stunt rules don't require a muffler, but almost everybody uses one of some type, and they all help. Some more than others. Wonder why that is? Yeah, me too.
If you're around electric stunters much, you'll notice they make a bit of a "bark" on hard corners, and it can be louder on either inside or outside corners, depending on whether running LH or RH prop rotation. There are various theories about what causes it. I tend to think it's blade flex or flutter, because I don't recall it happening when they were still running IC props on E powered planes. But then, I'm often wrong, according to my wife.
Steve