I would say don't mess with success! Others closer to your geographic area may have better opinions, though.

I am running the same scheme as you are doing, albeit I am only one year into being a pondmeister. I run one diffuser in winter, but in 3-4' of water to keep the immediate shoreline ice-free. I am pretty sure that one shallow diffuser still circulates the entire BOW to a constant temperature.

My only deviation is to turn off the system from 3pm-7pm when the electric company charges a higher rate!