It's not the key thing anyway, from my perspective. The key thing is his journey from seeing replicants as being something other than human that can be destroyed without a second thought, to being people who are ethically at least his equal if not his superior.
Suppose you had a character whose job was to hunt down and kill runaway slaves in the antebellum south of the USA, and the story depicted his journey from seeing them as something other than human that could be destroyed without a second thought, to realising that they were just as human and worthy of life as he. Would it really
make no difference to the story if he were white or black?
This isn't a wholly satisfactory comparison, because you don't need elaborate tests to detect a skin-colour, but I think it still illustrates the point. The realisation that people like you
are just as human as you, and recognising that people who are not like you
are as just human as you, are not equivalent moral judgements, in my opinion. The latter requires a greater stretch of emotional imagination, and that is why I think the moral core of the film is weakened if Deckard is not human, but Ridley Scott says I'm wrong.
I don't personally see how the addition of that dream sequence would change one's interpretation. Maybe someone could enlighten me.
explanation is that the dream is ties into the little origami unicorns that Gaff makes and leaves around. The implication is that Gaff knows about the unicorn dream because it is part of memories implanted into Deckard in the same way that the memories of Tyrell's niece were implanted into Rachel.