While gasoline leaves the carburetor as an atomized liquid, some or all will flash into vapour in the intake manifold, depending upon manifold vacuum and air temperature. As gasoline vapourizes, it cools the fuel mixture and the intake manifold. All OEM intake manifolds have a hot spot in the intake manifold to compensate for this cooling effect.

At full throttle, manifold vacuum is low and air flow is high so gasoline can enter the combustion chamber as an atomized liquid rather than vapour. Atomized gasoline displaces very little air. Because engines need as much air as possible in the combustion chamber (ie, volumetric efficiency), this helps the engine to make more power.

Propane expands 270 times going from liquid to vapour. I'm not sure how much gasoline expands when it vaporizes. In a gaseous fuel engine, the fuel displaces some of the air entering the combustion chamber so it will produce less power than a gasoline engine. If you're not running your engine at full throttle, you won't notice this effect.

On the street, a properly tuned propane engine will be very responsive because it doesn't require an accelerator pump and there will virtually no fuel mixture variation from cylinder to cylinder.

A simple performance improver for an Impco propane carburetor is the Impco VPV (Vacuum Power Valve). This device allows the engine to run slightly lean for better fuel economy and then enriches the fuel mixture when vacuum falls below 3"Hg.

As for liquid injection, I believe Icom NA has the only system available in North America. I think their focus now is on fleets and I'm not aware of their JTG system for retrofits. If they did, if would only be for EFI engines. Roush had the JTG system on their trucks about 10 years ago. See Roush Propane F150.