Ya know, after giving it much contemplation, I think maybe there really needs to be a differentiation made between direct-drive pumps and magnetic impeller drive pumps, as I think we are beginning to mix apples with oranges a little.
As far as I know, the vast majority of magnetic impeller drive pumps we employ in aquariums have a more or less free floating impeller and are essentially frictionless using the water as it's coolant & lubricant and the electro-magnet in the casing is switching at 60 cycles per second regardless of what the flow rate is.
It simply does not make any sense to me that the speed of the impeller as effected by flow rate via valves is going to have any impact on the amount of current being drawn.
I can not see any direct link between the two, the magnet is switching at a constant rate regardless of the flow rate.
Now the heat generated is being generated by the current in the electro-magnet, and that is a rate generally set by the cycles of the AC
current. (remember we are talking small aquaria pumps, not pumps to power a fire sprinkler in a high rise, large variable speed pumps as mentioned in the article linked or similar type systems.)
So with that in consideration, the heat generated should also be relatively consistent.
Flow rate will influence the subjective heat felt by virtue of the design of the pump being water cooled/lubricated.
The math really is simple, (I think) lower flow rate=longer contact time with a given volume of water as it moves through the pump and around the pump housing if submerged=greater the subjective
heat transfer to the water.
I say subjective because the cumulative heat transfer/build up in the system would/should be relatively the same because more heat overall isn't being transferred, it is just raising the temp in one section higher, IE; the water in the sump with restricted flow may be hotter than the display than in unrestricted setup, but the overall heat exchange is generally the same given all other factors are in common.
simple way to test it is go restrict the flow through your cars radiator by half and see if it gets hotter or cooler.
I hope that all makes some sense. :p
now a direct drive pump with a fixed impeller if blocked or restricted too much will transfer that need for more energy to the pump and cause more heat build up.
Simple test, go jam the impeller of a Little Giant pump and see how hot that bugger gets. I've seen them almost catch fire because of a jammed impeller, but then again they are air cooled.....