I don't know if you did this or not, I don't remember seeing it and I am not going back over 50 posts to find it. Have you done a proper compression test? Both hot and cold. Have you set up vacuum gauges on it to see how that are acting?
I did a dry compression test (no oil added) just to give me a general idea of any possible valve problems. I did note that the most forward cylinder on the left side was the weakest at about 190 psi, the rest more more closer to 196 psi. I only tested the left side since that is the side that seems to backfire or pop in the intake manifold under full load in OD gear starting at about 45mph and RPM's just below 2,000. Once the RPM's rise high enough, no more popping is heard in the intake and the bike runs very strong.
I also did a vacuum test on both the left and right intake manifolds using standard automotive vacuum gauges (not sync gauges). The right vacuum gauge needle vibrated, as did the left, but the left gauge (for the left intake manifold) vibrated a hell of a lot more (needle oscillation). I am not burning any oil and the engine only has about 39,000 miles, so I doubt there is anything wrong with the rings. However, given the corrosive nature of modern fuels (BTEX and Ethanol), combined with poor storage and maintenance practices of the farmer I purchased the bike from, I wouldn't be surprised if one or more of my valves are sticking, likely an intake valve.