My 2 cents on this great subject....Gas has an octane of around 87 and needs a compression ratio in the 9s to extract the energy efficiently. CNG has an octane of 130 and needs a compression ratio in the 12s to extract the energy efficiently. Propane has an octane of 110 and needs a higher compression ratio higher than a standard gas engine to run efficiently you will see a loss of MPGs and horsepower. With CNG conversions you see a loss of horsepower (10-15%) and loss of MPGs (~12% city / 7% highway).
I think most internal combustion engines become more thermally efficient as the compression ratio is made higher. However, it's the fuel's octane rating that limits how high the CR can be made otherwise the engine knocks due to detonation or pre-ignition of the fuel. I like to look at from the standpoint that higher octane fuels allow spark-ignition (SI) engines to have higher compression ratios. And higher CR makes engines more thermally efficient. Obviously engineers also have to worry about pollution in addition to power/fuel efficiency.
The Ford 3.7L V6 has a compression ratio of around 10.5, which seems about average by modern standards. With gasoline direct injection some modern engines have considerably higher CRs.
Regarding lower power, I'm under the impression that comes about in part due to the gaseous fuel reducing how much air the engine can breath during intake stroke. And if so, that may make it less powerful without being less efficient.