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).