Just remember, Forza Motorsport is a Microsoft game, so they will (and can) pay out the ass to have exclusive rights for any car or maker they want. I fear this will happen with Porsche as well.
It's not about FM getting the car exclusively, it's about Sony not having it in their game.
Then again...
PD does has an established relationship with Nissan, so that means that infiniti is in that as well, so we may not have to worry about any of them being exclusive for FM anytime soon.