What if the switch happened the other way around?
Let's say that Travolta started out as Castor while Cage plays Archer at the beginning of the film. Do you think it would've been better or worse than what we've seen in this movie?
My estimation is that the scenes where the villain is pretending to be the hero (played by Cage) would be less subtle, while the scenes w/ the hero pretending to be the villain (played by Travolta) would be less theatrical. And that may cause a significant disparity in the portrayals.
Then again, both actors were working closely together to make sure that their performances matched. So it's possible that we still would've gotten close to the same improvisations in the entire movie.