Its funny you say that because I literally was thinking the same thing-
I think is pretty smart with their packaging though. Why inflate the price of the car when A) most owners aren't pushing their cars (especially a non-PP car) and B) Ford knows full well that those who will drive their car hard will also have no problem modifying it. I think there is a certain expectation that the mustang is somewhat of a blank slate.
Personally, I think the 255 series tires on my '12 GT+BBK are fine with regard to width. For me, I found that replacing the PZeros with different tires really helped traction. But then again, I don't track my car and I don't have any power mods. With just springs/dampers, the car is a surprisingly effective package when driving on back roads. I think if I was to go the '15 GT route, I'd opt for recaros+PP and keep everything stock except for the dampers/springs.
If you want to drive a really under-tired car, go take a spin in a Mercedes C63 AMG. I have to try to lose traction in my car, but the C63 squirmed and slid if I even breathed on the throttle!