We find our 2004 Sienna is a good starter, even when we forget to plug in the block heater. Our coldest experience was a few years ago, staying with friends near Edmonton overnight to catch a flight early in the morning. It was colder than 40 below that morning, Sienna was plugged in and started without difficulty. However, the big back door would not open. We crammed our luggage in from the sliding doors and by the time we got to the airport (nearly an hour), the door did open.
Your scan gauge -2 reading is probably accurate. I doubt the block heater has a thermostat, but that is certainly an appropriate temperature. No point in heating it further because the engine starts very easily down to -20 and we don't plug in at temps above -20.
Incidentally, if it ever gets down to -40 again, remember the transmission - the block heater keeps the engine warm but does nothing for the transmission. The vehicle should not be simply "warmed up" and driven onto the highway at that temperature. Drive very slowly to gradually warm up the transmission before going on the highway.