Jet's are very inefficient at LOW r's.
Jet's are very inefficient period

The higher the RPM the engine makes it's top numbers after about 5200 RPM (varies with different mods) the less efficient the pump becomes, strictly due to cavitation.
In most applications (exclude full on race stuff) you need to have the engine matched to the pump (or vice versa) otherwise you are wasting your time.
If you need to turn 6,000 RPM to get the max HP/torque and have a hydraulic camshaft and it's not a roller cam, 1st, I need to see this focker

2nd, a B impeller at 6,000 R's will eat up a buttload of HP (like in excess of 700 HP) and sustainable torque will be hard to come by (meaning it may never get there.....).
If you can get some specifics (a dyno sheet would be ideal) on your build, someone on here
uh hum Tom/Josh
uh hum can point you in the right direction....
I have owned/driven a bunch of jet boats, only one ever turned over 6,000 (and was worth a damn), it made over 850 HP and was capable of over 110 MPH but only weighed in at 1420# full of fuel (lets just say
not made of fiberglass) and it turned a stainless steel AB Berkeley in a heavily modified pump.
Take the time to do the research, it will pay dividends in the end...And probably keep you from spending money doing stuff twice.

Hope this helps...

GT