I think the chances are actually a lot higher than that. I really can't see Apple issuing a recommendation for cMP users to buy a card that won't allow for FileVault, Recovery, etc. It's one thing for users to buy those cards on their own without Apple's blessing. But if Apple is actually going to recommend card(s) cMP 5,1 users to buy I think there is a strong chance they will figure out a way for those cards to be usable in the EFI environment.
I could be wrong of course, but their wording "recommended Metalcapable GPU" gives me hope. I just can't see Apple recommending something that provides for a neutered experience on their Pro-level desktop (or any of their devices really).