Of course, because the boiling point at high altitude is considerably lower, your water will boil sooner but it will take your food longer to cook or rehydrate.

I've gotten around the canister problem in below freezing weather by putting it under my puffy jacket for 15-20 minutes before trying to start it (it is rather a shock to body temperature, though, for the first few minutes!). I use a small piece of CCF under the canister to insulate it. And I use a windscreen about 3/4 of the way around, which helps keep the canister warm but not too hot. I also had to put my mini-Bic lighter inside my jacket to warm up before it would work!