Well, I JUST asked some French friends who own a dive center in FP this exact question! Me and my husband will get Rescue Diver before we go in Nov. 2018 and I was asking our friend if SSI would be just as good as PADI in the minds of the French diving authorities who define equivalents with the military-esque French system and like to tell us PADI-anything isn't worth much.
I've lived in France 13 years, speak French, my husband too, and we've dived in French Polynesia in the past. (I guess, point being, I 'get' the French, and it mostly drives me up a wall.) That time with just Advanced OW they put us on the beginner boat since we 'only' had 300 and 200 dives each and.... actually the reason was that "we couldn't go deep."
So turns out that my friend suggested that PADI and SSI are same-same to the French and so Rescue Diver with either would probably help us achieve our goal of not being left behind. But then he spent a whole paragraph saying if you get anything, get the Deep Dive certificate! I had never thought of that! I don't need to go any deeper than PADI prescribes, but the thing is that in French Polynesia they limit you to 29 meters, period, if you don't have a French dive certification, or more than PADI Advanced OW. I HOPE Rescue Diver will be plenty, and if we are limited in our dives on Fakarava (we are now at 450 and 265 dives) I'm going to be really ticked.
So I'm thinking, heck, why not go for the Deep Dive cert while I'm working on Rescue Diver. It certainly can't hurt to have the knowledge from the courses! But you never know when diving in France (or a place like FP where French law presides) - they do like to establish and follow strict rules. Anywhere else in the world they will look at your credentials, ask about your experience and number of dives, and then dive within reason on recreational dives. Only with French divers have we heard lots of discussion about how deep they went, or how deep they are allowed to go.... And finally, in Rangiroa we were out in open blue water so the "deep divers" from the other boat were about 20 feet below us looking at exactly the same thing! I probably would have hovered a bit above anyway just to make sure my dive lasted longer. Let's hope Rescue Diver is plenty enough to do all of the diving we'd like while there.