I can see your point - I wonder if you can get a compression tester that can be zeroed for your elevation?
Still, there are fewer air molecules per unit volume at high elevation, and thus the loss of power/compression in internal combustion engines. How we measure the true air pumping performance of an engine remains the question - since engines are just big air pumps anyway.
Steve H. - KE0KNH
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner