There are some really interesting MPG numbers in the boat test section on the Suzuki website.
If you go to the boat test section and look for a pontoon boat (as close to a MacGregor as you can get as far as prop etc goes) you can see what they got for MPG at different speeds.
Going faster does increase the gallons per hour but because you are also doing more miles per hour the overall increase in fuel consumption is not as much as you might guess.
Of course in our waters the currents also play a huge role … if you time your passages to use the current then going slower will conserve more fuel. If you are fighting the current then going faster might actually be more efficient.
This might be a good topic for a club night.