This would be great if it weren't for the fact that it plugs into the wall at night. The electricity that's charging those extra batteries has to come from somewhere. It's getting the standard Prius 50 mpg from the gasoline, and any extra mileage is coming from a power plant. Calling it 80 mpg is misleading.
Electricity plants are several times more efficient at energy usage a car. That being said, the charging of batteries to drive a car has it's own inefficiencies. I'm not sure whether this technique is more or less efficient in the end.
Although if we got the power from nukes, then we may not actually have better energy efficiency, we may though reduce oil usage.