i can't speak specifically for the 450 but both my 03 ranchers and my 05 foreman run exceptably well at up to 10,000 ft. the plugs get a little sooty but it is better than being too lean. going to paiute trail in one week and i'll have all the hondas over 11,000 ft. and don't expect any problems. i would say that if your atv never goes to a lower altitude, then rejet for where you are at, but optimum jetting for the rockies may leave your atv too lean at lower elevations and that could harm the engine.