Well, I can't speak of total specifics, but I do know that the Cobra has had the 13" rotors since 1994 with twin piston PBR calipers up front. The rotors became 13" Brembos in 99, from my recollection. The rear calipers are single piston and measure (I think) 11.65 inches.
As far as the fit on a 15" rim...I doubt you'll get a 15" rim over the 13" brakes.