You will still be controlling your AFR with your foot/AFC on a 12 valve. Don't install the kit for power gain. By the time you're making enough extra power from LP you'll be detonating and blow your head gasket.
As far as installing it for fuel economy, you're simply trading one fuel for another and will still require the same amount of BTU's to get you where you want to go at whatever speed makes you happy. The BTU's from LP or CNG will cost less, but you have to figure out how long it will take to offset the cost of the kit and the headache of keeping two fuel tanks filled.
This^^^^
I ran LP for a couple of years. It didn't smoke more. Again, you're controlling the AFR with your foot/AFC on a twelve valve.
If you get an LP tank at your house you can install a wet line on it and fill your vehicle at home for the same price.
I'm not sure I follow you on your first statement. If you open a CNG valve to air and provide fire, it will burn. The tank will not explode because the fire can't follow the fuel thru the valve where there is no air, but the fuel outside the tank will burn provided that there is enough fuel.