I use the Fia in the winter months in the north also. As of November the cover goes on and stays on as long as I am not towing. It keeps the engine compartment nice and warm and the intake air temperature is not even affected
I use the Scangauge II and have observed great MPG results of up to 3MPG gain with it on. The engine temperature stays right on 190F and never is any problem. The MPG savings would pay for the Scangauge.
The ecommoders (club that watches mpg's) suggest to everyone to block the grills of their vehicles summer and winter and use the electric fan more often. The advantage is the fan compared to the little bit of air is pushed in while driving.
Never place any cardboard in directly front of the AC,Rad and air cooler, it will not alow the fan to do its job of cooling the system if required. On my truck the fan never comes on while having the block. But one never knows when it will be required.
Look at the size of the bars in front of the new SD trucks. The rad has to be large but the advantage is the size of the fan.