I can't believe nobody has mentioned the fact that at higher altitudes there is less of a gravitational force. Less of a gravitational force means the disc isn't accelerating back towards the ground as fast(F=ma). Therefore if it isn't accelerating towards the ground as fast (in comparison to a lower elevation) it will spend more time in the air. This will allow your disc to travel further before it hits the ground. In actuality though this decrease in gravitational force at higher altitudes is probably pretty insignificant...
Lol! Sorry, but at high altitudes discs drop like dinner plates. The lack of air has way more of an effect than lower gravity... And I can't jump any higher either.
I've played at a pretty steady 5000-6000 feet quite a bit after moving from New York. Discs are more understable, and don't glide as much as they did back east... But it's not a huge difference.
Jump up to 8,000 feet, and it seems to get exponential. I've played Estes Park at around 10,000 feet, and it was comically frustrating. I lost a ton of distance, and my discs were taking off left as fast as they could.
In my head, I expected to pick up distance when I moved to the thinner air, but after playing at more extreme elevations it's the exact opposite.