Just to necro an almost dead thread and update the masses, here are my findings from Winter Park, CO. The main difference is in the bittering hops. The last PA and IPA that I made were barely bitter. The IPA can barely be considered an IPA, except for the fact that it was dry-hopped.
The evaporation rate does not seem to be too terribly different. I always have a few gallons of sterilized water on hand anyway.
I actually find the same amount of sugar is over-carbonating my beer, which makes sense with the PV=nRT equation. Halve the pressure, and the resulting volume is twice the original amount. Luckily I have kegs, so I can slowly bleed the extra pressure. I have had to significantly decrease my priming sugar.