If the dealer normally participates in X Plan, I do not see why they would say no for just a GT? Maybe right now they would just becuase the 13 is brand spanking new, but in a month or two when every dealer has some 13's I doubt a dealer would say no to x plan.
If it were me going in there trying to use the XPlan and they said no, I would say 'See ya' and go to another dealer
They would make more $ selling it at sticker than they would at X plan (get very little back from Ford). If they have an ADM of $5K I'm pretty sure they wouldn't take X. However IMO they would be dumb not to take it but some *edit* MOST dealers get greedy . . .