Seeing as how every iPhone has got a new CPU since the 3GS, Apple has a history of updating the iPhone CPU too.
Again, Apple has a history of never providing the iPhone with a better SoC than the iPad too. That's my whole point. That's why a die-shrunk A5X makes most sense.
Either way, Apple will have to break with history one way or another. Be it with the die shrunk A5X (iPad refresh mid cycle with it ?), using the same A5 in the iPhone or providing the iPhone with a better than iPad SoC.