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John,
  No, the pressure differntial isn't the same at all altitudes.  At sea
level, there should be no differential.  As you climb, the pressurization
gradually creates the differential between cabin and ambient pressure as it
attempts to keep the cabin at low (or no) altitude while the outside
pressure goes down.  The cabin pressure gradually increases until you reach
max differential pressure, at which time the cabin altitude begins to climb.
Not being a IV-P driver, I don't know the max differential or the altitude
where it's reached.  I know that in a typical airliner, the max differential
is usually around 7.2 psi and the cabin altitude gets as high as 8300' when
the plane is at it's max operating altitude.
  Skip Slater

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