If I'm not mistaken that's a flat bottom aluminum hull, yes? If so, unless you're loading it with a heck of a lot of gear it's a light hull, compared to a fiber hull of same length. Obviously decreasing HP you'll decrease speed. But unlike V-hulls, a flat bottom doesn't need to "get on plane", which would be a concern with decreasing HP with a loaded down 20' ft fiber hull. So if you're ok with less speed, I don't forsee a drop in 25hp being a cause for performance issues. Also, I reckon the johnson 115 is a 2 stroke. If so you'll notice a difference in low end power with the 4 stroke, which notoriously have less than low end than 2 strokes.