Due to the thin air the turbos will be working harder to maintain the intake manifold pressure requested by the ECU. There are factory failsafes that keep the turbos from over spooling, but constant use at high altitude will mean that those turbos are spinning at much higher RPM than at sea level. This could lead to decreased turbo life.

The 4.2 will be down on power due to the altitude, but this will actually help with the trans issue as it will be seeing less power and torque.

If you do some searching on the net you can find calculations to determine how much power the 4.2 will loose at your altitude. The 2.7t should be able to maintain rated power. I'm not sure at what altitude the stock turbo will run out of RPM and not be able to maintain full boost.

'95 S6


'90 G60 Corrado 2.0L 16v Supercharged