"Robert Swinney"

I would like to know if there is an easy "rule of thumb" that
addresses this and other differences between the performance of compressed
air and steam at the same input pressure.

One of the delights of early/modern steam technology was the indicator--done up
with literature and hardwood case--that produced a stylus's readout of engine
performance. Perhaps you might use one (or a modern substitute) to compare the
steam/air results on a given engine.
Frank Morrison.