The last I heard from Heath Lowery in SD, (who has the green-colored Kermit The Frog Aviomania single place open/enclosed gyroplane powered by the YG-4 engine), he was changing to longer primaries and secondaries than what he initially installed on that machine. He said the original exhaust basically allowed idle and then full-power, with little mid-range performance. He had rather short pipes.
I went from a home-built muffler on a longer pipe pointed aft along the keel under the prop (at the end of the regular Yamaha pipes) to using the original, heavy muffler (approximatel 20#). The resulting quieter exhaust tones enabled me to hear the intake being louder to my left behind me, than the new exhaust noise from my right while flying. It ran fine all the way up through an extra 500 more rpms here locally (200' ASL).
But when I took it to Rotors Over The Rockies in Utah, where the density altitude was +5K' ASL and higher, those extra 500 rpms only gave sputtering/ stumbling. I've gone to just straight pipes, which sounds about like the original loud exhaust note of that home-built muffler, and have back the crispness in all throttle responses. Tested by flying it quite a bit @ Alvord Desert dry lakebed in far SE Oregon (@ 4K' ASL), and it performs like B/4.