Here's what we know, that as of right now this is a RUMOR. It's a rumor because we have not heard or read a confirmation on this from Screen Gems. We have refrained from commenting on casting until we've heard official news from the studio. I have always pictured someone like Alex P as Jace. As I said on Twitter, Jamie is not who I pictured as Jace, BUT that does not mean Jamie or whomever is cast can't become Jace on the big screen. I'm not big on comparing things to Twilight, but a lot of people complained about Rob becoming Edward and then ended up loving him for how well he portrayed him on the big screen.
MM's is a team divided, Sophie has said for awhile now that she would love to see Jamie play Jace. I will say that just because Jamie or whomever is cast as Jace is not who I pictured, doesn't mean he can't do a great job with bringing Jace to life. I'm going to trust Screen Gems knows who will fit the part well.
Either way, I am anxiously awaiting some sort of casting news. I can't wait to see TMI on the big screen. Please remember to be respectful in your comments :)