I just took a road trip from LosAngeles to Denver. Our Stock ('cept ITG filter) ZTS felt really weak for about a day of driving at altitude. By the time we left 6 days later we had no problem handling the Vail pass (11,000ft+). Just an occasional downshift to 4th to keep from dropping below 65mph.
The quick way is to reset the ECU when you get there (ie disconect the battery).
Car will adjust, but you may not Driving at high altitude is like driving on hot summer days, the performance generally sucks (depending on how high you go). This summer I travelled to a high altitude + hot place and that allmost ruined the enjoyment of driving.
Dense pine forests somewhat compansated though (=oxygen rich air)