There is definately a point at which the pump becomes "inefficient", but like rear end gears in "car speak" the more the horsepower/torque you apply to the pump, the more pump (gear) you can pull.

Generally speaking it is seldom IMO to see a pump turn more than about 8,500+/- rpm before they start the cavitation game, but really depends on the setup, some of these race pumps get pretty exotic.

A lot of times the engine doesn't have the balls (or impeller to big for engine) to pull into its peak torque, so you end up leaving a little something on the table.

Again IMO...