A second issue you can get with over-motoring a jon boat is trailering. A larger motor will put more weight on the back end which could mean little or reverse tongue weight. The back will sway if you don't have enough tongue weight. The rear weight may matter in the water too, and cause the front end to ride too high if you can't adjust down far enough. The difference between a 25 and 30 might not be enough to matter much all the way around.