If you took the super dry mountain air from CO and simply compressed it to sea-level pressures you'd need like 100+ octane fuel, but I'm guessing where the OP now lives the air actually has some humidity to it...?
All depends on how the carb reacts to the extra air density. Austin TX (where the OP is now) is a mix of dry and humid weather. Being fall now, I would expect he is getting the worst case situation if he is running on dry days.
Interesting you mention the humidity though... one rally acquaintance had a turbo Subie motor run fine on an aggressive tune at 3-4k ft in WV in hot, humid summer weather. Fast forward 3 months to a cool, dry day in October in upstate NY at around 1k ft and.... kablooie! The engine management was right on the far edge and had no margin for the lack of humidity. It was tuned in humid NC summer weather ...