Grant Erwin writes:

I don't see it that way. If the spec isn't about CFM of pressurized
air, then why would the numbers differ at all for different output
pressure?

Because the pumping efficiency necessarily degrades as the output
pressure increases. The fact that the 5 hp example Coffman cites only
decreases slightly in rated CFM from 75 to 125 to 175 psi output
exhibits exactly this performance curve.

And why quote CFM into different pressures? Why quote
pressures at all?

Because different applications require different pressures, and
efficiency is higher when pressures are lower, and efficiency falls off
as pressures increase, so you want to set the tank cut-out as close to
the application pressure as you can.

Those numbers make sense to me as CFM of pressurized air. If you need
more volume than 14 cfm, you have to use a regulator to expand the air
downstream to get more flow. If you set your pressure switch to pump
your air tank up to 175 psi, then you have the headroom necessary to
regulate it back down to your desired end pressure but get quite a bit
more volume.

Absolutely impossible. You can't make a round-trip on pressure (pumping
in excess of the application pressure and then regulating it back down)
without it costing you in performance and efficiency, although other
goals may justify it. Doing what you propose will *decrease* the
output, not increase it.