I doubt Turn 10 (and PG as a result) would take exclusivity rights. They've seen first-hand how awful it is for devs to get a sublicence with the FM4 Porsche pack.
Yeah, I remember the shit they had to go though for the Porsche pack. I thought the LaFerrari was exclusive to them for awhile but then again, Forza 5 is barely getting the C7 Vette.