We don't need to.
Extensive testing by motor companies and racers has proven all these facts years ago.
I know people from Michigan who went on a 10 day trail riding adventure vacation to Colorado this summer...taking their own bikes... and they described the richer running characteristics they had when they reached high altitude... less HP in general though.. because of the thin air.
And you describe "less dense air creating less pressure drop..sucking less gas"
Can you show that there is LESS pressure drop at high alt than there is at low alt?
Maybe the pressure drop at both alt's is the same?
Though I have been told that this richer/leaner condition is small (few of those guys of those guys bothered to re-jet)... but the HP loss due to thinner air is more noticeable