Blimey I am astonished that you manage to get 36 knots, thats going some. You could change to S/S which would then let you raise the engine a hole but I am not sure the benefit would be worth the cost or trouble. The higher the props the easier they will ventilate everytime you go over a wave and the props come near the surface and with twin engines thats a lot more often than a single.
Big gains in speed require hp and fuel. There was a Delta for sale on Hayling Island with a 150 V6 merc on the back end a few years ago. That would have been very quick (and scary)