It is very important that the engine runs at, or near max RPM @ WOT. Check your manuel or with Mercruiser for what the max RPM is for your engine. If an engine is running 500 RPM to slow @ WOT you could cause some serious problems with heat and carbon build up. You may want to try a 22 pitch 4 blade Bravo I prop. As far as a flame arrestor, the K&N will be the best for efficancy and HP gains. Don't get to excited, dyno tests of motors with a stock flame arrestor verses no arrestor have only shown a 1-2 hp increase. With a 1-2 hp increase you probably won't see any noticeable speed increase.