Engines that have turbos are less affected by altitude changes because they build pressure above atmospheric. The reason the air is thinner at higher altitudes is because atmospheric pressure is lower. If you're boosting, the turbo takes care of that problem, for the most part.
Just get it tuned right. It might lag at the bottom end, but once you're boosting, it's not as bad. Contact a performance shop in Colorado (Denver maybe) to see if they can give you some help.