try running the motor at different angles of attack. Just play with the switch that lowers the motor while your running flat out. See if theres any diiference at a higher angle of attack.
Is your motor a long drive? We use a Merc 115 hp on a small T Bird and it planes out and runs the boat at a top spped of about 35 knots . (We cruise at maybe 25).
Does your engine fould much? Is your mix right? can you use hotter plugs?
Id start with the angle of attack , maybe that or setting the engine lower. Im n ot sure about the engine settings cause I bought my little T bird as a fixer upper project and the engine mount plates were already on.