I have a gheenoe 15 4 with a 1978 johnson 9.9 that has been correctly converted to a 15hp. loaded the boat runs at wot 21mph at 5850 rpm on stock 9.5x10 prop. With a second person on board I drop about 150rpm and 2mph. Would I see any advantage in a new prop such as a solas 9.25x9? The upper operating level is 6500rpm for the 15hp gear ratio 1:2.416.