If the speed and consumption were linear, then it wouldn't really be an issue In your example of 1/4 gal/hr at 2000 RPM, and 1/2 at 3000 RPM -- then say 4 knots = 1 gallon/knot, then 8 knots and 1/2 Gallon would be okay. Problem is that in flat water you're only getting 6 knots and you've changed your fuel consumption per mile. In heavy air, you have the same problem -- you just don't get anywhere quickly or efficiently.

In general, I think it's a function of the power curve for your hull and the engine. An easily driven hull makes a big difference.

Our 30,000 boat will do 7.5 to 8 knots at 2300 RPM in flat water (with a clean hull)...but at 2900 RPM it will approach 9 knots.

The consumption of fuel starts to approach 1-3/4 gal/hour though. Tooling back to 2,000 RPM, we'd do about 7 knots and the consumption would almost be cut in half (IN FLAT WATER).