Joe, the key to complete combustion of fuel in an engine is the fuel to air ratio and the timing of the initial spark or flame. In my plane engine I am running a fuel / air mixture that is so lean all the fuel is burned. I can duplicate the performance by adjusting the mixture to run richer, but it will cost 3 more gallons of fuel per hour. My point is that if 40% of the fuel is unburned then I would conclude the ignition of the fuel is too late and raw fuel is ending up in the exhaust, or you lack sufficient air to complete the combustion.

Introducing Propane may make less air for combustion available, or it may alter the timing of the combustion event so more burns in the cylinders and less in the exhaust.

I would pursue this further with people that really know, and not people trying to sell something or justify the fact that they bought a system. I would want hard data, along with a better understanding of specifically how more fuel being introduced results in less consumption. This may be the greatest thing since sliced bread, or it may be BS.