Living where 0c is the norm for 2-3mo and -35c is occasional, the radiator cardboard situation only helps the heat up time. I've driven my X5 at -25c and it will get the engine to normal operating temps in 10-15 minutes. The cardboard helps the speed at which the cabin heat is effective and I will usually put some in about Dec 1 and leave until feb.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)