It's the nature of the carburetor. These little diaphragm carbs don't have an 'air bleed' jet in the high speed circuit to keep the proper fuel mixture after peak power.
I put an air bleed circuit in a Zama to try it and it revved like a motocrosser. Definitely needed a limited coil to keep from blowing it up.
Seems most other 2 strokes, bike,sled etc with a slide carb. The only thing keeping them from running away is load, then friction.
Where we can fuel a saw to not run away.
Have you tried tuning one on the dyno to find the peak power tune that way?
They are all set on the dyno, ad it has its own load and 84dl of chain.
The true max power does not change much. .05hp ish. From 1k extra rich to 1k lean.
The lean tune shows better power higher in the rpm range.
Other than maybe limbing i don't see any of the over rev power being super useful.
I like a tune you could process a whole bar length tree. Drop, top, and buck it all into firewood. 16k may be a little hot to do that with most any saw.
So is the 4
4 hp at 15.5k really going to do much? Not much tq at all at that rpm.