One more point that I think you touched on in your last post but I wanted to make sure it was not lost.

As you pointed out, energy use drops off as a cube of RPM so I think you will find that you will use the least amount of energy per GPM at the lowest RPM even if the efficiency drops somewhat with RPM. Assuming the maximum energy use is 3200 watts, here is my reasoning.

Watts / GPM = 3200 * (RPM/3450)^3 / GPM = 3200 * (RPM/3450)^3 / (.033 *RPM)

Watts / GPM = RPM^2 /405322.5

So the efficiency is not likely to roll off at a rate of RPM^2, so the lower the RPM the less energy is used.

Like I said before, if you operate at 750 RPM, you can do 2 turnovers in 24 hours and use the least amount of energy possible.