No longer own a 5.7, however my last 2 boats both had them. Boat weighed ~9000lbs and maxed out at 1.7-1.8 MPG. Once the boat was up on step, MPG didn't fluxuate much unless I absolutely buried the throttle.
Engine used about 14 GPH at 3800, 16 GPH at 4000 and maxed out at 20GPH.
FWIW: Merc makes a device called the Mercmonitor that can interface to any MPI built after 2002. It will tell you the exact GPH (or MPG if hooked to a GPS via your plotter). It can send all the engine data to a NMEA2K network to display on your plotter. It has an "ECO" function that shows you the best throttle and trim position to maximize your fuel economy. Plus, it can send all your engine faults to your plotter -- pretty handy device and pays for itself fairly quickly. I had it installed on both boats with the MPI. Extremely simple to hook up -- simply connects to the ECM pigtail that will be hanging on the port side at the rear of the engine.