Not much. But, as far as powerheads go, the maxi jet is a power hog. A korlaia, Tunze, ect that has much larger flow (1200+) operates at 8-10 watts. That maxijet runs at 28w for 265 gph...not alot in the grand scheme, but still plenty a huge amount over other technologies.
Any system that utilizes an energized magnet is an on/off principal, unless there is feedback from the resistance against the impeller.
Larger pumps that are shaft driven may be able to reduce dynamically, but I don't see how a small scale magnetron is able to.