I have seen a few topics about replacing an old one-speed pump motor with a two-speed motor to improve efficiency (here and here for example).
I came across this website that said the following:
I was going to replace the motor on my max-e-glas with a two-speed, and I wonder if this is an issue. I did a quick search, and there are two-speed max-e-glas type pumps out there.One of the main purposes of a volute casing is to help balance the hydraulic pressure on the shaft of the pump. However, this occurs best at the manufacturer's recommended capacity. Running volute-style pumps at a lower capacity than the manufacturer recommends can put lateral stress on the shaft of the pump, increasing wear-and-tear on the seals and bearings, and on the shaft itself.