Say Davham:
I would bet that DA is correct that the Bisson muffler is knocking off power. Your own tests show the same thing when you compared open exhaust to the Bisson. For comparison, I have seen a Brison 3.2 turn the Mejzlik 22x10 @ 7100 rpm with B&B muffler. It's reasonable to expect the DA to be in the same range with a free flowing muffler. Others have posted here on RCU that the Bisson mufflers knock off 500 rpm on other gas engines like the ZDZ-80.