I think I remember reading the real hurdle in our boats is not the horsepower, but the [small] size of the pumps. the HOs aren't really all that much faster than the standards.
it's like sportbikes/cars: at a certain point, you need a large amount of additional power (or aerodynamics) to overcome wind resistance in an attempt to gain a few MPH.
From the data that I have collected, the 21-24 footers with 155mm pumps really start to lose efficiency over 200 hp. Anything over 250hp is really just a waste.
There are options to make them 160mm pumps.
http://www.greenhulk.net/forums/showthread.php?t=187481
Some quick notes concerning:
1. Not needed unless you are supercharging that 1.8
2. Will increase your efficiency etc as the more veins will help straighten the flow and will still flow more due to the size of the pump.
3. Remember how Yamaha went to a 160mm pump? It was right after these guys made this announcement. And only on the SVHO models if I recall correctly.
4. If you want more than 250 hp, say.... like 300, then in our boats my general graphs would suggest that you need more like a 170mm pump (remember, at 7000 rpm) which is about 6.7 inches. For reference, 155 is 6.1. Doesn't seem like a huge difference, but I'm reading the data, and trying to make equations work out of it, so that's the difference, and that is really the minimum pump size for efficiency.
If you REALLY want to talk efficient and hp/speed numbers, well, then you need to have a single pump (as dual pumps create more friction in bearings, side walls, etc.) and then the size will depend on how much horsepower. In all reality, you don't want a supercharger because it has parasitic loss. I'm wondering who will take the supercharger off the 1.8 sho motor and put on a turbo? They do it on the ski's from time to time!
http://rd-performance.com/turboKits/yamaha/fzr-sho-svho.asp
But at that point the efficiency thing is kinda gone....