With most Fords, you can enter "engineering" or "test mode", to see the actual coolant temp. You hold down the trip odometer button while turning the key to the "start" position, until it reads "engineering mode", then you can toggle through various things until you get to coolant temps. Then, you'll be able to get a handle on what your coolant temps actually are while driving.
We had some rare "cold" temperatures for us, high teens, and what I noticed is that around town, there was no difference--but on the highway there was a 2-3 C difference (lower). Still, the coolant never got below 81C once warmed up.
As others have said, you could zip tie something to block off the grill if you're doing a lot of highway miles in really cold temps, but it's probably not necessary. I'd be inclined to cut up a sheet of ABS plastic though, versus cardboard....just seems less combustible!