what are the dimensions of the boat? what type of hull is it? if its around 30", youd probably be better off with a 700SC on 12 cells.....

you should be getting better results from twin 540's than a single...

you would get better results by just running the engines without vector steering and using a rudder...w/ vector steering, you lose speed in the corners since one motor isnt spinning...although i would keep them counter-rotating, as this gets rid of prop walk, but if you had both spinning in the same direction, it would just increase prop walk...

you could be leaking from the prop shafts....you could also be leaking from the back of the boat....there could be a small gap or crack where anything is screwed onto the hull....it could also be coming through the hatch...there are a large # of things that could cause water to get through....

one thing though, does the prop shaft vibrate alot? if so, the vibrations could be cracking the hull where it goes through the transom, allowing water to get through...