Nick,
Going back to the original question for a moment -- While you say the Vne on the S-76 is somewhat arbitrary, is there not an aerodynamic reason for the decrease with altitude and temperature? I'm flying an A++ and find with a light load at altitude I have to back the power off a good 10%-15% from the torque setting at max gross weight to stay under the placarded Vne. It seems like there's more to it than merely correcting IAS to TAS.
Thanks,
-Stan-