From ClippedCub
They would have tested this configuration to Vdive at altitude, but the dynamic pressure is a lot lower up high as opposed to the 5,100 ft race altitude, and the test would have been done at 1 g.
Speeds such as VD are specified as Equivalent AirSpeeds in the standards - hence altitude should not be an issue. The exception is when mach number is a limit - MD rather than VD.
Was this aircraft actually tested to a declared VD?
As a matter of interest, does anyone know to what extent these aircraft are tested against standards? As an experimental aircraft they are obviously not expected to meet all standards (eg FAR23), but is there a set of minimums that are applied?