Have to say, I've ridden all over the country on my Yamaha and a little Austin Healey (43 HP).
Colorado was the prettiest state....but you do lose a good chunk of power at altitude.
Timing and fuel mixture are set for your normal riding altitude, any changes will reduce power. And it is noticable.
Nothing to worry about, just plan your riding accordingly. IE, Don't pass going up a long hill at 8,000 feet altitude...you might not make it in the distance available.
I never had a problem....just slowed down and enjoyed the scenery...
djd