If anyone has a definitive answer, then dont expect to see it on this forum!....much like myself, if I find a significant performance gain, I'm not going to broadcast it.
I dont run 2-stroke, but if I had to guess, I would say the older engines are more broke-in than the newer ones.....much like Hondas, once you get about 50 hours on them and go to the second or 3rd piston, they literally take off.
Continuing from the above point, as an educated guess, I would say the key area of performance is the cylinder pressure, which is a factor of the barrels cylindricity deformation under heat, once its been heat-cycled so many times it will stress-relieve itself and start to stay round when its hot, unlike when its new......thats where my money would be, but what the hell do I know!
I would also say the break-in procedure used by your dyno-man when its new is a big factor in getting the piston-rings to bed in correctly.
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