How would a stock KLX do in Colorado at high elevations?

 

You might be OK. They come stock with a #118 jet, so you may not have to touch it. You could also try to contact a kawi service dept in the area and see if they jet them differently as part of their prep.

 

I rode mine up to around 10.5k - 11k elevation completely stock, no problems. Yes, the smaller factory jetting is probably better in the thinner air. The only thing I would consider is dropping to a 13tooth up front, or increasing the rear sprocket if you are going to be riding trails/steep climbing off road.