I wouldn't do it if I were you. The boat is rated for 10hp for a reason -- weight of the motor
, stability under power. I've found that the maximum rated hp for ribs is really very much the most power you can use.
We had a 15hp two stroke on a rib
that size on the old boat. We figured that since it was a two stroke, and the rib was rated for a 10 hp four stroke, it could handle the weight. Well, it couldn't, and couldn't use the power, either. We sold the 15hp at a loss, changed to a 5hp, and were very happy.
On the new boat we have an Avon
340 rib with its maximum rated hp -- a 25 horsepower two-stroke. It's just the right amount of power -- you just about need full throttle to get it on a plane with the maximum rated load on board.
If I were you, I would stick to the rating.