what paul (pagz) and i were refering to is that possibly your piston rings could have been damaged by pre-ignition or detonation,
there is a few things that cause this:
to higher compression ratio
and to advanced ignition timing
another can be from build up of carbon etc in the combustion chamber.
but these all have to do with fuel quality, that is why i bought it up.
ignition timing is normally limited by fuel quality, the better the fuel quality the more advanced you can run, hence more power,
as a general rule you want to be as close to detonation/pre-ignition as possible without it actually happening.
there is more things that can cause this but these are the most common 2.

i only put that up for an explanation for you of why we said this.

but you engine builder sounds clever and he is probably correct that it is from a worn engine.

and as paul said, is it all 6 pistons with damaged rings?