I'd never really thought about deep frying, because I don't do it myself, but I found this:
http://www.geniuskitchen.com/about/high-altitude-adjustment-for-deep-fat-frying-964
Sous vide is cooking at a specific temp, so altitude might affect how quickly that temp is reached but not how long it cooks otherwise.
Water boils at a lower temp at higher altitudes. I cook eggs a bit longer here in Tucson than I do back at sea level in BC. But I hadn't really thought about oil temps before.
Edited to add: I should say I BOIL eggs a bit longer here. Other cooking methods may not change the same way.
