I usually don't go beyond 400 or 600 grit when I sharpen but I do use a leather strop and some stropping compound at times. I think edge geometry has a lot to do with edge retention and I suppose it is possible that by going for the mirror polished edge, at a microscopic level, you are also changing the edge geometry, thinning the very edge. So, it does make sense that the mirror polished edge might be the weaker of the two...but it also seems that initially, the mirror polished edge would be sharper (due to its thinner edge).