As I build my e-pump setup I was going to test the pump power consumption at the full range of engine RPMs. This would allow you to see how many watts the pump uses at idle, cruise, and high rpm acceleration. After that, the plan was to start optimizing it and reducing power consumption as much as possible. I'm pretty sure there is a lot of room for improvement here over belted systems. Of course, you are paying the alternator efficiency penalty (if you use an alternator) by using electricity vs mechanical power.
Adding an e-pump does have a couple benefits that you simply can't get with a belted pump.
1) Heat with your engine off. This is a fairly big one for me as I live in a fairly cold environment in winter. I sometimes have to leave the engine on to defrost the windshield, and heating the cabin is challenging and annoying with engine on/off cycles. The e-pump completely solves this.
2) Preheating. The e-pump gives you more options for preheating your engine. You can circulate the coolant while it heats up. This could be used for cabin preheating, or transmission preheating as well. Or, you can do like the Prius does and use a thermos to store hot coolant. Or, you can use a crazy big heater and preheat the engine in 10 minutes like Oil Pan 4 has done. Or, you could perhaps try to circulate coolant through a solar hot water collector like
NoD has suggested. It just adds a lot of flexibility for preheating and using and moving the heat you have.