Understood. But I really am curious how your battery will do. The engine bay still get pretty hot (much hotter than ambient). Not sure the difference in ambient vs under the hood is relevant?
Why do you think every summer the local parts store parking lot is full of cars waiting for their batteries?
I used to live in Vancouver, BC. I took for granted batteries lasted 5-6 years until I got to TX. I went through 3 flooded lead acid batteries from 2001 till 2008. So that averages out to about 2 years each battery.
Ambient temperature here in TX is easily 40c/104f. We're talking about 1-2 months of that heat. Overnight rarely gets below 86f. The heat radiating off the ground adds to the under hood temperature. Any flooded lead acid battery in this heat causes the acid to boil. Necessitating biannual refill of distilled water. If you fail to do the drill, the battery dies in 2 years or less.
AGM battery do not have actual liquid!!! You can flip them upside down. Hence they last longer, ie 5-6 years. Nothing to boil.
It's all chemistry, heat speeds up the chemical reaction in the battery and evaporation dries the battery.
How will Subaru's EFB battery hold up in the heat? That's an open question, there's not enough owners on here that have owned them long enough in hot states to provide a good data sample.
You can believe what you want regarding ambient vs under hood temperature. Just telling you the facts about what happens to car batteries here in the heat. My own personal experience with my 2001 Forester and seeing what happens every summer.