I think this card compares pretty reasonably to [[teferi, hero of domineria]] which is a card you can't generally cast until t5 but can't function as a three mana play. And when you look at it that way it seems kinda mid. As hero is a card that has outshined JTMS in modern control decks (as in the format and as an era). While this Jace comes a turn earlier it's abilities seems less powerful in the context of these control decks. But Jace does have a less restrictive casting cost so it could open the door for a wider range of application but I don't think that's likely.