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A4 Turbo at Altitude

Does anyone know for sure whether the A4  1.8 Turbo uses an absolute or
a relative pressure measurement to determine maximun boost?

Several months back there was a lengthy discussion on the list about
boost pressures and altitude.  To summarize, some turbo systems
determine maximum boost by measuring the difference between  internal
pressure and the external atmospheric pressure; these systems are
relative, and , since the external pressure falls with altitude, the
maximum power of the motor will decrease also.  Other systems have an
absolute pressure reference, and so the engine's power will not decrease
with altitude--given a turbo with the extra capacity needed to run at
greater than standard relative boost.  

Which sort does the A4 have, and if it is an absolute reference system,
does the turbo have enough capacity to keep full power up to, say, 9000
feet?  Since the V6 is noticeably affected by altitude (I drive one in
the mountains a lot) the 1.8T even stock might be a better car for high
altitude passing.  It all depends on the numbers, which I haven't got.