Assuming you win a game, they'll know which fireball variant you're using. I agree that I would still personally name either Channel or Mirror, but just pointing out the flaw in your logic. If I knew you were running fireball, I might put a second mage on that if I drew into it.
But can't they just do the same thing to Emrakul? MM on him still kills that win con anyways right? Hence the reason I was going to run 2x Braingeyser and 2x Stroke of Genius for the wincon. That way they cannot MM both and it is not a dead draw by any means, rather a combo enabler that allows the deck to draw Mirrors and Tutors that it would otherwise not see. So what if I need to make extra mana over Fireball or Emrakul, when the combo is working then you rarely fizzle anyways.