I have a 15hp Johnson, 2 stroke and the consumption is pretty much fixed at wide open throttle regardless of the "weighting" of the boat. it's about 1.5 GPH. The only thing that will change much at WOT is speed/ distance covered, but you will essentially be burning the same amount of fuel per hour regardless of the amount of gear/ people in the boat and such. Consumption is pretty much set by chamber volume and rpm. As long as there are no real changes to either, the engine will always consume the same amount of fuel at WOT (give or take a small percent for Gremlins).
I assume you will get similar "mileage" measured in the available number of hours of WOT burn. Keep in mind though, the actual distance you cover can vary DRASTICALLY with just about any change in variables such as wind, tide, weight, balance, tube inflation, piloting skills, and stock market valuations! (well kidding about the wind part. Lol).