Hello av8tor, You have come to about the same performance limits that my Gx 31 did.
You might gain a little more by cleaning up and enlarging the intake and exhaust runners. There might be a little to be gained in the cam lift and duration,although most of this gain would probably be in higher rpm's than we need for aircraft operations. You are most likely right on the money to expect substantial gains by increasing the compression. The boat guys do this by pluging the wrist pin hole with bearing material and redrilling the wrist pin hole 25 thousanth's lower on the piston. Maybe someone could cnc a longer rod?
I have three of these and would love to get just a little more power to justify the weight.
Other than that I beleive this is my favorite motor.
Motorhd