Having said all that, the Vmcg for certification purposes is calculated for the WORST CASE, and not the prevailing case, thus the official Vmcg published in the AFM will be the same irrespective of the Altitude.
But there is provision to be allowed to schedule minimum control speeds with variation in temperature and altitude, precisely so that you are not unduly penalised for unfeasible amounts of thrust at high altitudes.
Originally Posted by AC25-7A
If, at the option of the applicant, the AFM value of VMCA is to vary with pressure altitude and temperature,....
(Yes, I know that quote says VMCA...)
Of course, if the OEM didn't do so, then you have to abide by the "single value" rule...