Sam maybe you can go to .010 or even .008 but if you burn plugs it too tight. A higher head clearance will give you torque off the corner if your dragging a heavy boat. You may compensate with a bigger head volume. If your looking for more speed start at the back of the boat,prop and make sure the running surfaces are flat and sharp.
Your clearance in that GO is huge if its factory. Probably at about .025. A good clearance for you would be .008. You go lean when you tighten them up james, and they take a plug every time.
I would focus on getting the boat around the pond first, THEN worry about the clearance.