Darn, I hit the wrong button meant to start a new post. As far as horsepower goes If you go to 9.9 you will be almost 25% over the rating. I`ve done it as have others but I don`t think its a good idea. Based on the rating your boat is probably pretty small.If you can handle it a bigger boat will perform better horsepower for horsepower and feel safer.A longer wider boat will plane easier, handle easier,feel safer and generally run faster with a load.This assumes it is a basic boat like your present 12 footer.