Even that's a stretch. For instance, Halle was billed higher than Famke in every X-men film despite Famke having more screentime than Halle in most of them. Why? Because Halle was a box office draw. Billing orders, in cases like these do not always reflect the level of screentime an actor has. Quite often the top five billed will be the main actors followed by the biggest stars.
If anything the top five billing for this movie should be:
Hugh Jackman
James Mcavoy
Michael Fassbender
Jennifer Lawrence
Halle Berry
Hugh, Halle, and Jennifer are the biggest stars in the film and Hugh also has the added benefit of having a good amount of screen-time meaning that he may very well get top billing. James/Michael aren't box office draws but are the two main characters (supposedly) and so would be billed after followed by Halle and Jennifer the biggest female stars of the franchise, to round out the top 5 billing. This is how I would expect the billing of the film to be. Regardless of whether Nick has more screen-time or not I don't see why he would be billed higher than Halle or even Anna to be honest. He is about as big a draw as Shawn Ashmore, only difference being Nick gets more work in bigger films.