Alex I think you're absolutely right! The -500 was awfully Vmcg limited especially when operating to CAA limits (ie 7 knots x-wind used for calculating Vmcg). By reducing thrust to -22B values on short runways we were able to lift something like 20T more cos we weren't eating up runway trying to get to a V1 that was higher than Vmcg!!
I suspect that even using FAA figures there would not be much payload available at all on 6500' with meaningful fuel.
Happy to be proved wrong tho!