Hi
There are usually not many negative things when maxing out the HP on a boat. Maybe a few dollars more but I suggest that you consider the benefits of the 25HP before deciding. The 25HP will make the boat perform the way is was intended to work. It will get on plane in a shorter distance and in the most efficient way (I have experienced better fuel economy with a 25HP over a 15HP because the engine isn't working as hard) You will get the best speed performance and I know you indicated that speed was not a factor for you but its nice to have it when you want it. Just because it will go quicker doesn't mean you have to run it fast all the time but maybe if you want to get off the lake because of a T storm you might appreciate the extra speed. As I mentioned before your fuel economy should also be good if you don't need to run it hard. Another thing that might be a factor is the weight or load you have in the boat. If you only ever go alone with your fishing gear that's a pretty light load but what if you decide to add an extra fuel tank, maybe a battery or two and a nice bow mount electric steer/spot lock trolling motor? What about bringing another angler and his gear plus a cooler etc.? You might not do any of these things initially but down the road you may want to and the boat will be ready for that with the 25HP. I upgraded a similar boat (Lund) from a 15 to a 25 and the difference in overall performance was night and day. It also increased my travel range because it was faster and made the boat work properly. I actually traded in that new 25HP only a month old for an upgraded 25HP with electric start and power trim/tilt and I would highly recommend this as well. It is worth the extra money without any doubt. The power trim/tilt, it makes a huge difference in the way the boat works including being able to dial it in on those rougher water conditions. You will need a battery on board but that was also a bonus because I added running lights, a really good sonar/gps and a plug to run my radio... It goes on and on but at the heart of it is the 25HP.
Best of luck and enjoy that new boat!