You shouldn't be seeing any drastic change in power at any distinct altitude. I've driven my Xterra dozens of times over 13,000 feet, and in 1st-4Lo, it still feels almost the same as at altitudes closer to 9,000 feet.

Still, it has never not had enough power to drive up anything. Yes there is a decrease in power as the air thins out, but never enough to be described as huge.