Unless you have a way of proving the concrete board does add to the fire rating, I wouldn't rely on it helping. Use the rating that comes with the safe as they have been tested to that rating
Not sure if there is a Menards Hardware store around you but by me they have a large assortment of all types of safes and far cheaper. I've seen safe's on sale all over the internet so if I were you, I do more shopping first.
You are comparing it all wrong. The fire rating for sheetrock and cementboard is how long it will take to burn. Can cementboard reflect heat? Does it absorb heat? After a fire has been extingished, the contents of the safe can take as long as almost 3 days to cool. During the cooling period, the safes interior can continue to absorb heat.
So as I said before unless you have some way of proving it adds to the fire rating, I wouldn't rely on it helping.
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