What does it prove?
It proves that...
- Nothing in the control system or aerodynamic surfaces has stretched, contracted, tightened, loosened, etc. enough to cause the start of flutter. (I've seen that)
- The pitch control circuit hasn't done something similar so as to cause a reversal in pitch stability, which if it occurs, is likely to be seen first near Vne (the phrase CH601 springs to mind).
- Control authority remains at high speed, this is why Paul, Bob or Darrol, depending upon who checked you out from CAA, will have taught you to make small control inputs at each speed increment. Control authority can be degraded by cable stretch, something gone in the surfaces or wear in a joint. I've seen an aircraft lose aileron authority at Vne.
- The canopy is still stiff enough not to implode on you at high speed (I've seen that as well, quite exciting, fortunately in my case a bendy one so it just changed shape).
- Propeller control is still correctly set-up to allow you to avoid red-lining the engine (although I'm sure you are aware of the need to retard throttle towards the late part of the dive).
Regarding overstress, Vne is no more than 90% of Vdf which is the limit the prototype (and depending upon company procedures the individual aircraft) was taken before approval. Given wind energy ("Q") goes with the square of airspeed, you are at Vne only taking it to a state of 81% of the energy the aircraft's designed to cope with.
I've certainly not heard of an overstress, and hope that if the airframe was that degraded, it would have been picked up in maintenance and sorted before the air test.
G