Back in the late 1950's and 60's was when they first (to my knowledge) began playing with propane fueled vehicles. These were NA applications and the results of their toils left little doubt that propane has a higher octane than most gasoline blends. They were able to consistantly run higher compression ratios than was possible with the gasoline of the day, and this is when 103 octane premium was available from the pump.
Based on that, I would say there is no question it has a higher octane and like in blended gasolines, you will end up with an effective octane that is the weighted average between the fuels in use. Propane is reported to have a RON of 112, and a MON of 97, so it is quite similar to Methanol in those regards, being slightly higher in the research octane number.
It has very nearly the same peak flame temperature as gasoline 1,990 deg C for propane vs 1,977 deg C for gasoline.
The cooling of the propane occurs at the moment it flashes to vapor, this adiabatic cooling due to pressure drop and the absorption of heat heat of vaporization as the liquid flashes to vapor cools the propane gas to approximately the boiling point of the fuel which is about -42 deg C or about -40 deg F. Just like nitrous oxide cools the air charge, the propane will also, but only by dilution in proportion to their gaseous specific heat.
Propane gas has nearly 2x the heat capacity of air under the same conditions so -- for a back of the envelop calculation -- for every degree the gaseous propane warms up, it will cool an equal weight of the surrounding air about 2 degrees.
(someone who plays with thermodynamics all the time, and has authoritative numbers for the relative specific heats can work out the exact numbers )
In the case of Nitrous Oxide it will cool down the intake charge something like 70 deg F at normal injection rates, and I would expect that propane injection will do nearly the same -- perhaps 50 deg F cooling. BUT -- it has to be injected far enough ahead of the intake valve so the engine sees the effect of the cooling in the manifold in order that the VE of the engine increases. If the cooling takes place inside the cylinder, its too late to change the amount of fuel air inhaled on the intake stroke.
The true gaseous nature of vaporized propane certainly appears to help combustion.
For an example of the probable process, in research done of flammable dust air explosions they found that small amounts of flammible gas (like propane) added to the air, greatly speeds the combustion process as it adds energy necessary to heat the dust particles to combustion temp. I suspect the same thing happens in the engine as the flammable gas helps the evaporation of the micro dropplets of fuel and so speeds, and increases the effectiency of combustion.
I would say that a hybrid system of water injection with propane injection would certainly have some possibilities for that last little bit of power. The propane won't have the same cooling effect of fuel dumping, so the WI can provide that, to hold down combustion temps, and the higher VE due to the cooler manifold temps from the propane should push up power levels significantly. Propane is also much cheaper than high octane gasoline last I checked, since in this application it would be a dual fuel system where you only used the high octane fuel when you needed it. Propane of course like Nitrous has the advantage of being self presurzed so you only need a solenoid to turn the flow on and off.
The combination I always wanted to persue was a propane fuel enrichment for a nitrous system. That should be capable of melting your pistons before you know you have a problem
The down side is now you have to manage several high pressure systems and their controls. If some one came up with a universal switching module that had multiple trigger outputs to control solenoids according to manifold pressure, engine rpm and TPS, with a built in clutch safety and low fuel pressure and or blocked water flow safety, hybrid systems would be much simpler to implement.
Larry