Ken@Japan - But does't the Prius incorporate some sort of built in 'thermos', whereby the hot coolant is pumped into to preserve heat, then pumped back into the radiator/block when needed? If yes, does this feature work when driving between EV/ICE modes? Or does it only come into play when you stop? Or am I just full of "Hot Water"?

The FEH/MMH cools down rapidly after entering EV mode from the air passing over the radiator fins. Block the front helps considerably without a 'thermos' holding system.