Great explanation AZD. I would add that although there may be some reduction in heat when throttling the discharge, there is also a corresponding loss in cooling flow. IME, the loss of cooling is greater than the heat reduction and when restricted more than a token amount (my pure guess is ~20%), results in the pump running warmer. A great example of this is the use of MJ1200s in powering media reactors. As they are often restricted to optimize the flow through the reactor - the loss of flow (cooling) results in the impeller and shaft running warmer and can dramatically increase the amount and rate of calcium build-up and result in increased maintenance. Another option is to by-pass some of the discharge water back to the sump. Although you don't gain the energy reduction, the pump runs cooler.ALL centrifugal pumps can be restricted on the dischare side without harming the pump and it WILL reduce the power consumption. This only applies to centrifugal pumps and most all aquarium water pumps are centrifugal pumps.
Its hard to grasp but read up on Bernoulli's Theory and The Laws of Affinity. Basically what it is is that by restricting the flow you are reducing the amount of work being performed thus reducing the power consumed. It also usually reduces the corresponding heat the pump produces since heat is directly tied to power consumption.
I mentioned the above cooling issue as I've been running a WaterBlaster HY7000 for about the past year as my return pump and have had problems with it seizing about every 6 weeks due to calcium deposits in the bushing. I currently have it restricted about 30%, but am going to work on a bypass too see if it extends the time between tear-downs and cleanings.I'm waiting on a water blaster myself that's what's tarted this thread lol