I started thinking that if the regulator does not close properly, it should affect the low end as well, should it not? Yet it idles very well without any tendency to slow down. Also, when I took the glow plug, the element was a nice gray. I would not expect that from an engine that runs rich.
So what else could it cause loss of power at high rpm? A worn out ring? Compression seems good, though this being my first ring engine I do not have a baseline to compare to. Also, when it is warm it starts as well as when it is cold. In my experience with automotive engines,
warn pistons/rings loose compression as the engine warms up and the cylinders expands, making them harder to start...

Also, could it be the 15% nitro is too low? I will try to get a jug of 20/20 for comparison...

Alberto