Hmmm. From a scientific standpoint: Passivating the surface produces a very very very thin layer of chromium oxide on the surface of the stainless steel (converts available chromium on the surface to chrome oxide). Since the oxide is harder than the substrate metal, wouldn't this result in an increase in the hardness of the surface?
But since the passivation layer is very very very thin, this is probably not anything that can be practically measured, so from an engineering standpoint, there is no increase in hardness.