From my experience in trips to CO mountains ( 8k ft - 13k ft ) every summer for 30+ yrs. In the 70s and 80s on the old trials and enduro bikes, carbs required dropping the main jet size quite a bit. With the newer bikes with the CV (vacuum) carbs not so much. Each bike is different...KLX300 with CV carb needed a much smaller main jet. Don't remember exactly how much but was supprised how much it took. KLR with CV carb needed very little reduction, 150 to 148. DR350 with CV carb none. And really ran great. All had "opened" air boxes. XR650R with non CV carb went from 150 to 125 main. Don't know how much air box mod will help. You could lean idle fuel screw ( turn in ), might help idle but probably won't effect mid and high rpms much. Some people have reported no problems at altitude. I haven't had my TA up there yet ( next summer ), so I'm also interested to hear what jetting and mods the Colorado TA guys are running. If you're not getting knock or hot running, lower octain gas might help. Hard to tell with the alcohol blend gas we have now. Try to find non blend gas. Makes 3 to 5 mpg difference in my car. Not much way around some loss of power though, short of a turbo.