850 pax config? Yet to be seen.
You are right of course if you mean the current configs used out there. However it was used during certification flights and of course the evacuation testing done in hamburg. Actually evacuation trials were done after 71 seconds (iirc) and therefore they can increase pax load considerably in larger versions if airbus ever builds them, talk about a 1000 pax airplane.
If current operators put 400 less seats in there than they could it is their own decision and i would expect they have it calculated on that basis, but of course the per passenger fuel consumption has to be worse than in a high density configuration.