I think BvS will do big numbers.
It has a number of advantages in its favour :
1) MOS has already preceded it so the audience has some expectation of the tone and style of this new DC universe being introduced in BvS. With MOS, there were a few people expecting a Donner-style Superman but that should no longer be an issue.
2) It has not just one global icon in the form of Superman as one of the lead characters, but also another in the form of Batman. Even fans with no interest in Superman may watch this film because of that.
3) It has a strong returning cast from MOS, and also a very credible new cast in the form of Ben Affleck, Jeremy Irons and so on.
4) It has a villain - Lex Luthor - who is arguably more known to the GA than Zod was. Luthor is the iconic Superman villain above all others.
I look at Nolan's Batman films as following a similar template. Batman Begins is a brilliant film and it had a moderately successful box office, but it didn't do amazing numbers - just under $380mil worlwide. There are many reasons for that; you can blame Batman & Robin for leaving a sour taste in the GA's mouth when it came to all things Batman, you can blame it on BB being quite a different take on Batman than the kid-friendly Schumacher films which came before, you can blame it on Nolan being a relatively unknown quantity at that time.
Irrespective of those reasons, the film set up a new structure for the Batman universe with a new tone, a new style and a more credible approach with the word 'realism' often thrown into the mix. So when The Dark Knight was released and did it's amazing $1 billion+ numbers, the GA had an idea what was coming (and more to the point, liked it) and I think that had a huge impact on the success of that film. I see MOS as paving the way for BvS in the same way.
Plus, it was just a brilliant film too which led to great word of mouth. Whether this happens with BvS remains to be seen yet.