Saying somebody "will never be Brady" is an obvious and unfair comparison, nobody will.
Jimmy can make every throw, he needs to build confidence as a starter but the skills are obvious. BB wanted to keep him for a reason and since he left the Patriots this is the best team he's had around him, one of the most talent rich rosters in the NFL.
Also Shanahan creates offense through scheme, using his personnel to the fullest and raising offensive play calling to an art. Charlie Weis and his father used to be fun to watch in the same way. These guys do more than just call plays, they give their players in the best chance to succeed using misdirection and scheme.
Now that Shanahan has the horses (talent), that is a dangerous team.