ok, I'm gonna try here, The maximum output velocity of jet drives are directly related to intake gullet rake and volume, as well as intake area. Then the other variables, impeller efficiency, pitch, venturi shape, size and output area. On top of the drives capabilities, boat design is key, dead rise, how wet the hull runs...on and on.

The key principle and this is tough to wrap ones brain around is this, a boat or aircraft can only go faster if the output velocity is greater than the intake velocity. So simple... nope this means if the jet cannot accelerate the water faster than it comes in than you go no faster. With an impeller with a certain pitch, you can only accelerate water so fast. Since this is not a surface drive where the boat is riding on a blade, you put that prop in a tube and speed it up, eventually bubble will form on the leading edges of the impeller and your efficiency will decrease. These bubbles stay in the duct unlike a regular prop, so the jet stream aerates due from the impeller cavitating. Further more the only way around this, is complicated. Almost bordering on bioengineering. The intake and outlet of the venturi must be variable or the impeller must be variable pitch.

I'll leave it at that for tonight, but it's still not this simple at all. I've written this out so many times and it doesnt get any easier. This one is pretty good, at least to me. Try searching around, not much info out there on the theory behind water jets or should I say beyond the simplified theory.

food for thought.
http://en.wikipedia.org/wiki/Jetboat
here's a big one, but I recommend anyone interested in jet pumps to read through this whole thread
http://www.boatdesign.net/forums/showthread.php?t=11254


ooooo found something related a neat chart...notice thrust has to go to zero [:-]
So the next thought which you've already started thinking about is how to go faster......that second link goes pretty in depth with that