After 4k, you ought to be checking your filter and your vaporizer... and looking at your plugs for signs of premature wear or poor running.
LPG likes colder spark plugs, so if you haven't changed already, do so. Doesn't seem to do much for economy, but it helps reliability and power.
If your car is distributor type, you can add a little more advance on LPG... we had to do it with a piggyback. But then, my engine doesn't seem to knock even with extreme ignition advance on gasoline. If yours does, better to not take the risk.
I wouldn't muck around with cooling and engine insulation much. LPG does absorb a lot of heat in the state change, but that occurs in the vaporizer, which only helps with the coolant temperature, which is already managed through the thermostat and the radiator fans switching on and off. What you have to be worried about is the temperature inside the combustion chamber, which is hotter because it doesn't get any of the benefits of the cooling from the phase change, gasoline enters the engine as a liquid spray, hence the cooling effect as the heat makes it turn to vapor. LPG enters the engine as vapor already.
This is why LPG tends to burn out spark plugs that aren't "cold" enough.
LPG will only be superior to gasoline overall when the next generation fuel systems with liquid injection come online... (I doubt yours is if it's only $900...) as those will definitely run cooler than gasoline, with attendant benefits in power and possible parity or even superiority in economy... but I have yet to see a system like this on the market... still hoping...
Then again, as we both know... with LPG dirt cheap, it's still a better alternative, as long as your local filling station carries it.
