I have the exact same thing you describe when reaching a certain rpm in top gear. Also noticed one cilinderhead is significantly cooler than the other one after a ride. I also hoped it would be a carb issue since I've been reading the cx500 carbs can be prone to clotting etc.
I have my block totally free and stripped of paint and I am also contemplating wether to break her open or just leave it. Also could not verify if the distribution chain has been changed or not, I heard it was a "thing" on these models.
Anyway, good luck! I will be following your progression.