It can affect it a bit at high altitude, usually it requires a longer sitting time (hence, where a very insulating cozy becomes very important).

9K is not as huge of a issue as say 11 or 14K, it isn't huge till you get over say 10K. I have a chart on how water temps go down for boiling/related to altitude on the old website. I need to see if it ever got moved over.

The foods affected most are the harder to rehydrate items - brown rice, whole wheat pasta, thick cut veggies. Items like ramen, freeze dried veggies, mashed potatoes, soups, couscous are not going to be affected - they don't need boiling water temps to rehydrate. Much of the dry foods can come back with cool water if one wanted.