The props that come with most RTR boats aren't the best in the world so replacing it with a good one will see a huge improvement in speed. The Kyosho prop has too much pitch for its diameter and that prop they throw on most of their line-up. You should be able to spin an Octura X-440 2-blade even though it sounds like what you already have, it isn't by a long shot. If there is any slop between the prop and shaft that can be easily fixed with a tiny piece of brass tubing. A Prather 215 or 220 might work also.