Um... you're talking about electronic effects, right? Very small changes in composition most certainly can dramatically affect electronic characteristics -- the crystal structure is not as important as electronic structure. (I know very little about metallurgy -- you may well be right in this specific case -- but I think it is silly to dismiss a concern about the effect of changing the composition because, allegedly, the crystal structure is the same. If that were a reasonable argument, wouldn't it negate much of what is known about semiconductors?)