Of course rom has a direct impact but the biggest impact is when the boat reaches hull speed. That's where you have to burn a lot more fuel to gain very little speed till the boat is on plane. That s what really matters, understanding that once you reach hull speed, either you get the thing fully on plane or you stay at hull speed.
The 1400 rpm and 20 gph is spot on for my 12 71"s we are just slightly over hull speed at that point about 10kts on 66 ft w/l