It really depends. Persona 4 has sold almost 2 million copies between its two versions, but that's probably more a testament to the dedication of its fan base than anything. This is the first time that a main Persona game that is neither a spinoff nor a remake is being released only on an aged and obselete system, so that will help it as the thread has pointed out.
I also think Sony would do both Atlus and From many favors if it were to release a 350 dollar PS4 bundle with Bloodborne and Persona 5. Anecdotally, it seems the biggest problems with Souls and Persona is convincing people to play them, since the 'hardcore-ness' of both series is offputting.
It's also speculation, but I do expect we will see a sizeable boost in Persona 5's Metacritic score compared to the previous games, because critics haven't been as universally in love with the series until after P4:G. If its quality is comparable to P4's, I expect to see a 94-96 Metacritic score, which will also help its sales. At that point, it may even be a darkhorse candidate for GOTY.
