Well, "you gotta pay to play", and electricity production aint cheap anymore.
A 2 speed could work on the water feature , i.e., rock water fall, etc.
But to apply it to a shear decent...I dunno, I dont think your going to push enough volume of water when you drop to the lower rpm, so I would work with a high efficiency single speed if it were mine.

I have a 1.5 hp single speed on my 5 ft shear decent, and I control the volume of water on it with a valve.
I do notice on the electric bill when the wife and kids have been using it.

Pentair has a new Variable Frequency Drive pump that I think over time will have applications in water features. No doubt it will probably carry a hefty retail price.