Thought I'd throw my own experiences in as well, 5 speed R34 GTT sedan getting about 34ok's for a tank of BP 98. Basic mods, cold fed pod filter, 3" turbo back, FMIC, stock ecu, 12psi. Engine and transmission conversion through a number of less than qualified workshops (including 'Nizzpro' when they were at Ashmore). Thought it must have been a wire or 2 not connected as I got the car back and the speedo, air con wasn;t working and was running only 5psi and felt real flat. Turned out was a knock sensor not connected (thanks Chiptorque) Made 164kw and ran a lot better so I thought fuel economy would improve as it was no longer running on a 'safe' tune.
NOPE, 320/340 a tank so no change at all. AFR's continually bounce between 13.5 and 15.8 cruising on the highway (AEM UEGO gauge).
But on a positive some gas rebates still apply and liquid injection technology is getting good gains over petrol.