It would be interesting to do your test with a few different props. See if your spike hits at the same time. As for the practical RPM range, the only way this engine is going to see anywhere near 16,000 is with a much smaller prop, much much smaller.
It doesn't sound like your stand has any dampening at all. Most any engine that is mounted to metal has some sort of isolation, so your test might be better served if you at least went to a composite mount on an aluminum stand/frame..
If you think about, even if you take an aluminum mount and mount your engine. What are you mounting it on? Wood, that's right. Unless your using an aluminum airframe(kidding, I know your not) , full scale planes that are of a metallic airframe definately have dampeners, whether it be rubber, or something that has dampening characteristics somewhere.
The OS 60FP is/was one of the smoothest bushing motors on the market. Another interesting test would be to test a .61FX with the same prop you tested the FP.
Sounds like youv'e got some cool things going, and a good job that allows and supports a good hobby. Thanks for sharing your finding, looking forward to more when you find something out.