it adjusts to keep the AF ratio right (although the tables are always a little conservative, so you will usually run a tad more rich, nothing excessive though). However, thinner air has less oxygen, so despite the computer correcting the fuel table for less air, there is still less air to burn in the same volume, so unless you increase your displacement (ie bigger motor), or force more air in with a forced induction, you are always going to lose power. Even FI cars lose power, and nothing automatically restores it.. The "upside" is, with less oxygen, you can run lower octane on the same AF and spark advance.. Not much consolation, since you lose power either way, and don't fuel up in a high elevation area then drive down to sea level if your tune is for 93 octane..
Sold: 2006 GT Premium Vert, 5 spd, Tungsten Grey
New toy: 2001 Porshe Boxster S