I live in the desert at around 1200 feet. My XT is jetted slightly rich to compensate for the drilled stock muffler (PO did it, but didn't rejet) and to avoid engine seizure while running in 115 degree temps. I have ridden it up in the mountains close to 10,000 feet, and it is blubbery as hell, but still runs. If I lived up there, I would have to down jet it, but as things are I just have to live with it.
The problem I see with nitrous on a bike, even if it is just there for high altitudes, people are going to use it for performance, and quickly blow their engines. I am an amateur drag racer, and run a small block Chevy in an '89 S10. The engine is well built, but built more for durability than absolute max power, as I can't afford to keep rebuilding it. Seems like every time I go to the track, someone running a Japanese car using nitrous blows an engine. Stock engines are built to barely handle stock power.