Thanks for sharing all your info.
I have not read all your posts, so please forgive me if I ask what has been before.
As for the mintor Vs OS compression – you cant really compare this by turning the engines over cold. Mintor have alu pistons in steel liners hence the design is such that the bore tolerances are different to allow for the piston to swell when the engine runs and heats up. Not sure though on the OS design though. i believe ring and ring land design also can play a part in this, such that some engines will have low compression until they run at which time the ring is forced outwards against the cylinder walls. i am no expert in this regard however.
What pipe setup incl header length are you using. Have you played with the header lengths. I have a Mintor 38cc on the test bench and brand new out the box after 10mins got 7600 rpm using a JXF / Xoar 19x8 wood prop. The JXF wood are higher load props than APC to my knowledge. This is also on the std exhaust system and not a pipe.