The best way to improve top speed is a planning wing...It attaches to the lower unit and looks like jet wings...you will plane way faster and improve your top speed...I used em with a couple of smallish outboards...
I have a "whale tale" planer on my 30 horse and I have mixed feelings about it It seems like it does get to planing much sooner that it did without it, but it also does not allow the prop to get down deep in the water and I can't get to top speed because the prop comes out of the water and cavitates. I take the "whale tale" off and I can go much faster but I don't plane as quickly.
Getting more power out of any motor involves:
a) making sure it's working normally (no bad plug wires, clogged injectors/carb needles, proper cooling, etc.)
b) burning the fuel more effectively (increase the compression)
c) getting the fuel/air mixture to flow better (bigger carb, freeflowing filter, etc.)
d) changing the power curve (swapping camshafts)
For the average person, apart from item a, these options aren't practical and it's just easier to buy a bigger motor.