YUP... I am that guy with the cardboard. It does help things warm up quicker, and keeps things warmer when you do get into EV mode in winter and stop creating any more heat. Heat to the cabin will drain your (small) radiator heat reserve very quickly, so I turn off the heater when I'm in EV mode.
I've been able to EV down to 0'F with the cardboard, but below that, it's a lost cause!

I know for sure that the A/C compressor does not activate when outside air is 32'F or below. And thus, the ICE would not have to run if everything else is warm. However, FOR SURE the 2005 model year will not turn off the ICE if the defrost is on. If the 2007 does, then that is a very cool update!!!

-John