The water pump is a simple centrifugal pump. A centrifugal pump at a given rpm uses less power with less flow (as would result from increased drag).
I would agree with you if the thermostat is restricting flow into the pump but not if it is restricting flow out of the pump. A restriction on the intake would lower the water pressure, on the output it would raise it.
But I do believe that some cars have the thermostat on the intake side of the pump now so that is a good point.