I am at 7500 feet and I have gotten some outstanding results and some no results. I have found that it appears to be more in the shipping route the eggs take rather than attitude. Eggs that come from Texas, Oklahoma, & California always seem to do better for me. I have gotten eggs from several different breeders in those area and I can usually count on some hatching that is providing the eggs were good fertile eggs to start with.
I rarely get good hatches from any eggs that come from south east US. Once in an awhile some slip in, but mostly I get damaged air cells. I do surprisingly well with most eggs that come from the north east US and I get them in 2 days.
I can almost always count on not getting many to hatch from Kansas and Missouri and it takes 3-5 days to get here. You would think with it being so close to Colorado that you would get 1-2 day service and great hatches. I do have an exception on the Kansas rule currently as I have 24 turkey eggs that took 5 days to get to me and 20 of the eggs are growing nicely. I got some bonus chicken eggs from the same source and 10 of the 12 eggs developing.
Anyway this is what I have found out from watching where the eggs are coming from. I have started to watch this closely when I am thinking of spending big bucks for eggs.