Yeah, I looked at the audio using spectral analysis tool
https://www.audacityteam.org/ . And at a shade over 12k it was still had some four-stroking. Under load as you knock it under 12k it stops 4-stroking, and as said earlier it will produce usable power down to ~9.5k.
I did my analysis using this clip. Personally I think it sounds
very fat, and I'm loading it hard through a cross-grain stack.
https://www.youtube.com/?watch=vTf52xz25RQ
In the first cut at after 30s it starts to 2-stroke, as I lean hard on it. At 42s I almost bog it down (audacity says it's turning at 6k - thus almost stalled)
I'm into all this nerdy stuff, I'm afraid
science rocks....