re:  At altitude that air density becomes less and less in relation to
the amount of  fuel going through the engine, so fuel air mixture goes
to the rich side ...

Think the air mass sensor compensates and this is only true with
carburetors.

re:  i've definitely noticed increased gas mileage at altitude ...

Less air, compensated by air mass sensor equals less fuel (and less
horsepower) and certainly should equal more mpg.  It also means less
compression since air is less dense to start with, meaning less octane
requirement.