You really aren't comparing apples to oranges. Nitrous is an oxidizer, basically it adds oxygen to the combustion process. Propane is a fuel. Propane has a higher octane rating than gasoline (I think its about 110), and its cheap per gallon. The reason you don't see it used very often is its a bulky system and the parts are expensive and kinda rare. Propane is stored in a big, heavy tank as a liquid at high pressures. It is then fed into the motor as a gas. To do this, you have to use a mixer which is basically a carb. There really aren't any performance mixers out there, so you have to tweak an available system from a few companies that don't like selling to individuals or sharing with tuning secrets. It can be done, but Nitrous is FAR easier to get running.

 

That's not that big of a decrease in air temp to justify the complexity. If you're going through the trouble, add water, alky or even nitrous. My friends SVO Mustang used to drop air temps during a 1/4 mile pass once the alky kicked in. He ran 7.80's at 94mph in a full weight SVO mustang with no intercooler at all.