I would not be concerned about running the 92 octane, if you build the motor with the same cam, heads, cubes... but add another point of compression and corespondingly higher octane fuel you will gain some MPG from the increased compression, it makes it very close to the same price to operate the car and you get a little more power to boot. MPG is mostly based on part throttle cruising and higher compression really helps part throttle efficency.