Even with a 600 speed motor and 7 cells I doubt you will hit 20 mph. My boat (turbo Vee2) is lighter and shorter and went about 20 mph with a 600 and 7 cells. I put in a AYK 540R motor from fun rc boats (19 turn and it's plenty fast now for it's size. Now it goes around 20-22 mph and still decent run times.
http://www.funrcboats.com/