I just don't see how the franchise is going to go from 350M to 1B. I just don't see it happening, X-Men isn't a billion dollar contender anymore, IMO. They've saturated the franchise too much for too long.
The saturation of it came from the abundance of spinoffs. And while all of them paled in comparison to the main series, financially, only 1 of the 3 was wholly unpopular with audiences.
The main series films have always been successful, growing in gross with each film, regardless of critical response, and grossing closer to the (then nearly impossible) billion mark. The spinoffs so far have been poor at the box-office because of general lack of interest in spinoffs. Not because of disinterest in the series.
Having a proper "main series" film again with the OT cast, as well as the FC cast playing a role, is enough to get butts in seats. Having it based on a popular story, and (at least from the looks of it) being a damn good movie is a plus.
The only that could possibly hold it back are the smaller, but still significant, releases like Dawn of the Apes, 22 Jump Street, and "Dragon 2" that come out in the following weeks.
However, something that will definitely help counteract anything detriment those might have is that about half (maybe more) of international markets don't get DOFP until July, as originally schedule. And some of which that don't get it until then are significant in determining WWW gross.