Stanard 156 brakes on the v6 have a single 55mm piston. These calipers have 4x 40mm pistons [I've just looked it up again]. Obviously that's a big increase, but these calipers are just the same as the standard 330 brakes which will work off a standard master cylinder [i'm not the first v6 owner to have done a big brake upgrade]. Is the GTA master cylinder any different to the 2.5 V6?If the standard 4 piston calipers for 156/147/GT have 40mm dia pistons (I don't know the exact dimension, but it's close to that) and you have fitted calipers with 60mm pistons, then you have more than doubled the piston area of the front brakes (Area = pi R squared). Assuming you haven't changed the rear calipers, you must still be close to doubling the total piston area, so the brake pedal travel is also going to be nearly doubled (excluding any slop or free play in the system, which won't be much).
I'm confused. It should be fine. Lots of others have upgraded to Brembo 4 pots without swapping the master cylinder or having excessive travel...