If the ICE in factory config does not get above 207F (mine), then I personally would not want to run it at 234F with the assistance of grill covers to get higher MPG.
Don't forget Paul also lost the transmission on his car and had it rebuilt. Maybe not related but maybe higher operating temps take their toll faster on parts. Not proven either way but something to think about.
I want to clarify some facts so we don't have miss information out there.
1. I have 215k miles on my CMAX and my BlackStone Labs oil analyst is better than new.
2. Failed transmission at 98k miles was caused by bad bearing, not heat. I have about 20k miles more on "New Trans, not rebuilt" with no problems and with Gill Covers on almost all the time.
3.I have twice hit the 246*F overheating alarm driving up I-17 from Camp Verde, AZ and it hasn't hurt the ICE. I was trying to find out what happens when you hit 246*F and found out, the alarm will startle you for sure and your automatic response is to let of the gas which solves the problem. Turning on the heater will lower temps quickly too. I think the best mpg's happen at 210-230*F. I think the ECM might start to add fuel to cool ICE much above 230*F. The thermostat is fully open at 202*F , FORD said operating temp range 202-212*F depending on conditions.
4. There is no reason not to keep the center Grill Cover on all the time, it only lets in air to the radiator and A/C, the lower grill cools everything just fine. The fan is very powerful and can pull air through A/C, Rad., Trans. and inverter coolers with grills blocked. And you still have the upper grill that puts air into ICE compartment. I don't notice much difference with covers on and off with A/C on. I have two oil pan heaters which I think add 5 *F to overall temps do to lose of cooling surface.