I'm not sure if the ECU adapts to altitude if you're at a fairly consistent elevation - It's when you're a low altitude dweller and you change to high that the mix gets starved of air and there's a noticeable drop in acceleration, fuel economy and performance. My guess is that the ECU would adapt with time by adjusting the mix. If you'll search around on the web you'll see postings by people crossing the divide or doing other high altitude trips relating the issue as a loss of power.

-- John