Well been there and done it. We use to live in Flagstaff, AZ and have Daughter and grandson in Phoenix so the worse climb temp wise is coming South on I-17 from Camp Verde for about 10 miles. The only time I have got the overheating buzzer twice with Grill Covers on. This is at the top of white band on Smart temp guage,(246*F) as I was reaching the top of the climb. This not a problem with lower Grill Cover removed. Just thinking you have a NRG so you could EV the tough climbs so you could use Grill Covers all the time. Do you have a ScanGaugeII or OBDII gauge to measure WT, TFT and AI temps?
I have one, but I don't use it. I used in on my Escape Hybrid, but just never really felt the need with the Energi.
The HVB doesn't last long on highway uphill slopes. I generally shoot for using both ICE and HVB in those situations. But on a long uphill, even 5.6 KWH doesn't last long.