I have a 96' johnson 25hp on a 14' jon boat and avg when i cruise around 22- 23 mph. That's a good speed you'll probably get the better gas mileage on rather than having the throttle mashed the whole time. I've had mine up to 27.5mph but i haven't had it on a calm lake yet. I just got it about a month ago. That's hauling *** for a small jon boat though i think. For what it is and what it's meant for. You can't beat them for the price.
It's worth playing around with different props if you can find someone willing to let you borrow a prop that fits you motor. I wasn't wanting to increase my MPH, just my fuel economy.
Just put a 9 7/8 x 13 pitch SS on the yamajammer 25hp and I'm topping 32mph GPS with just me and a solid 30 with two people. It would hit 31 but wouldn't maintain it. I've also boosted my MPG by over 10%.