So is the reason for the larger ST spare tire (over the standard Fiesta) because of the larger brakes? I'm wondering if the compact spare (non ST) would fit on the rear of the ST? If so, seems like if you got a flat on the front you could jack up one side, move the good 17" rear to the front and put the compact spare on the rear. Just thinking of ways to maximize the space back there by running a non-ST compact spare.
Also thinking that one could make a custom fiberglass/carbon fiber tray that form-fits over the standard ST spare. Maybe some 1/2" foam or rubber adhered to the bottom and auto carpet on top.