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...