First of all, Porsche calipers are Brembo. Second, I know many people have replaced the crap calipers with Brembos, but how do you know what the pedal travel is after the change?
Third, it would be quite hard for the Porsche calipers to have a larger piston area than the GTA calipers, as the GTA ones have 38/42 pistons, maximum size Brembo uses is 46, likely and from what I know, Porsche Brembo calipers use smaller pistons (36/40 or even smaller) so they will be even closer to the original calipers' brake fluid volume required to move the pistons.
Fourth, any idea how much of a difference that "slight difference is m/cyl size" makes? Quite a lot, from say 25mm to 28mm, there is a lot of difference.