I'm running completely stock in Denver. I haven't had any problems at all with altitude. It seems to compensate for wherever I am at (usually between 5,000-12,000 ft elevation). If you plan on riding twisty mountain roads where top speed is not an issue I would actually go up 2 in the rear sprocket to gain the power on the low end. My personal preference has always been to go up in the rear rather than down in front. I am basing this on nothing whatsoever, but in my head, the tighter the turn up front the faster the chain will wear out and the angles will be further off for the chain slider and might rub.