The demo does not need to come out right now, I personally do not think it would be a great idea since waiting for too long can also have the opposite effect on hype and eventually people will just talk about games that have come out, instead of a game that is coming out.
Although I do think they need to have two demos out, personally if they can dominate Tokyo Game Show coverage (Which is entirely doable) they should release a demo soon after the event since I'm sure there is bound to be a lot of buzz, then closer to release, sometime like January, they ought to release another demo.
Of course this needs to be backed up by a strong marketing campaign too, which can go either way with SEGA.