Sheep country varies considerably with terrain, vegetation, elevation, etc. Here in Colo timberline is usually around 10,000 to 10,500'. Doing the same hike you mentioned at 10,000 to 14,000 would be a lot tougher on legs and lungs than similar elevation gain at 6 to 8,000. Put a 60 to 80 lb pack on at 10 to 14,000' and you are talking a serious sheep hike!
I hunted goats this fall in Alaska....started at sea level and got my goat at around 3,000'. The hike up the hill was through a jungle of devil's club, alders and cliffs. My lungs had plenty of oxygen but climbing through jungle took a hours just to cover a couple miles. Add to that a hefty pack and I was certainly glad I was in Sheep/goat shape!
As mentioned earlier, sheep/goat hunting can vary considerably from 1 location to another....better be in "sheep shape!"