My stock 126 jet starts rich mixture bogging at full throttle in fifth at around 10,000 ft.. Not at full throttle or in lower gear it runs OK except for the power loss. At 12,000 it really runs out of power and anything steep and slow in first becomes a problem. I now have a 122 for Colorado, good from 7,000 to 13,000. (Some have gone down to a 118 since they live there) I also went to a 55 tooth for some extra grunt. That would be easier since you don't need to modify the chain. You might try the holes in the air filter cover method too.
Kinda depends on how much time you will spend at altitude in Colorado, I was at 10-12,000 only one day, so I just lived with it. One nasty short hill I just barely made it up by charging in first.... Five yards more and I would have stalled.
I'm hoping to get to the San Juans in early August for a week and I will definitely be re-jetting and going to the 55 for that long of a time. They got some 13,000 foot passes down there!