I think BTU per pound of fuel will give a better comparison. Alcohol has less BTU per gallon than gas, but has more BTU per pound because a gallon of alky is a lot lighter than a gallon of gas.

Propane won't do as well NA because you can't get enough of it in the engine vaporized and mixed with air(BIG expansion ratio from liquid to gas)to make serious power. One of those pesky laws of physics or thermal dynamics or Murphy's Law I'm not sure which, I'm not a lawyer.

With a turbo however you can compress the charge and make it denser, and because the fuel is a gas that mixes with air, you can compress a bunch of it. The more properly mixed air/fuel the more power. The potential I think borders on scary assuming the hardware could stand the strain.

Kevin