Most van's have some form of the AeroAcoustics APE system fitted as well. Cannot remember the supplement well enough if there was any variation in there for Vx or Vy, but could be a reason.

I've flown the van for an operator as well which had company Vy far lower than whats published in the POH.

ED hope that 206 had super good aftermarket baffling or a suitably inaccurate CHT gauge so you didn't know how hot that conti was getting!

If Vy decreases as excess thrust decreases, then why does the ROC decrease in turbocharged aircraft such as the GA8-TC going from 1000ft/min down to 600ft/min at 10k? Presumably the TIO-540 can maintain 38" MP at 10000ft...

The prop blades are like wings - drop the density and you lose thrust. Yes the CSU can make up for this by increasing the pitch (effectively increasing the coefficient of lift for the blade) but this also leads to an increased amount of induced drag on the blade.

makes sense. Big effect, but then I guess the thrust margin of 300hp hauling 1900kg is quite small so wouldn't take much of a drop in excess thrust to make big difference in RoC