Tim,
I have not had mine up real high to any degree; I mainly use it in the 100-1000 ASL range. Usually I am in the 100-200 ASL range, as I live in a BIG valley where I do most of my hunting.
Interesting though, I'll keep it in mind when I get up high next time.
I usually have an area mapped out on paper with yardages the evening or morning before the hunt, so it isn't as important to me to get a good reading in the mid day sun. This has screwed me before not doing so, but I always map a new area in the "optimum hour" at dawn or dusk well in advance now. The 1000-1200 yards this unit measures quickly in these conditions is plenty good enough for me at this point, my practice rarely extends past this range anyway, I just don't have the equipment or the practice that will give me any more range than this "yet". I will say, a Barr & Stroud would be very nice if my practice range was increased to 1500 yards and I became proficient there. Confidence in a first round kill would determine the range for me; I cannot usually see a spotter round's impact where I hunt.
When I can get 100% hits on a 12"x12" steel plate at 1000 yards I will consider moving out beyond there, staying within the kill zone on a moose might be possible then. Right now I can stay within the kill zone on a moose at 1000 yards without any problems, but don't have the accuracy to move out any farther than that yet, wish I did but that's reality.