Why people always say rx8 high FC? I can confirm that full throttle a 4G92 also will drink about the same volume. But when compare to Myvi or Kelisa, no doubt it is high on fc.
Convert to NGV is pointless as the queue always damn long most of the time (day time). I recommend you to do an engine swap. 1st thing to do is to remove all things from the engine bay and sell it. Then get the money to buy a KE70 halfcut and transplant the engine into rx8. After than, sell off the 18" wheels and get a set of 16" with 205 tires. You will then not only reduce the FC but also will have some money to compensate your fuel expenses for quite some times. If you think time is the problem, there will be an easy way to achieve your target. RX8 6MT has 6 injectors and 4/6AT have 4 injectors. The volume of the injectors are equally divide to front and rear rotor use. Unplug the rear set of fuel injectors cable connectors (3 injectors for 6MT and 2 for 4/6AT), remove the rear rotor spark plugs (leading and trailing) but let the coil remain connected (to prevent CEL), use silicon to plug the spark plug cable and make sure no water can enter. This is for make sure no compression at the rear rotor else it will drag the front rotor power. In this way, you will save the fuel by approx 40%, with the expense of 55-65% of HP loss. Exhaust note will be reduced by approx 50%, to approx 76dB. 0-400m drag estimated can be achieved at 31.7 seconds. This data was taken from a rx8 that with rear rotor apex seal blown but still able to drive with only the front rotor working.
Hope this will help you. Don't waste money and time on NGV.