franc, wow, to be asked by name, what an honor

As you state, air motors are not known for energy efficiency. However, the air storage mechanism is significantly more efficient than electrical batteries, and would be near 100% if not for some losses due to adiabatic cooling and minimum pressure differential in the regulator.

Your figure of 45% is probably cutting edge based on the work you've done, I have no idea how much a low budget motor could provide, unless efficiency and cost are loosely coupled in air motors. But 45% is actually very good compared to a gasoline engine.

I'm surprised that liquid CNG / propane would not be better used in this application.

On the other hand, adding horsepower to a car like this is easy, just turn up the pressure regulator :P