Mike, I remember reading a good quantitative piece in Shooting Sportsman that was similar to your findings (in the places that I remember). The similarity implies that there is some valiue in generalizing from your chart.

I've been using Lawrence Copper Plated 6's because I shoot muzzle loaders, and I can tell the copper 6's from the lead 7.5's without my glasses. But, it is a little discouraging that they are softer, as it costs considerably more!