I tend to associate Mile High with crowd noise and adverse weather more than thin air. From what I've seen, the thin air is something that only gets exploited on long field goal attempts, rather than offense. I'm sure John Elway was just as good at sea level. I also think he resembles a horse, but that's not germane to the discussion.
Anyone have any statistics on their kickers. I know Elam range in Denver is greater than anywhere else.