First off the front and rear calipers on the GT and V6 are the same. The only difference between the gt and v6 is the front rotor diameter. The GT rotors are 1" larger. Cars equiped with ABS/TC have no prop valve (this includes all gt's which have abs standard so no gt prop valve is available). the use dynamic rear proporting (DRP) which uses the abs to control the the rear brake balance based on the front/rear slip differential. This system is much more effective than any prop valve and can dynamically adjust to lood conditions and deceleration levels.
Non-ABS V6 cars do have a prop valve. The base brake balance is close enough to neutral, and the critical point is at .85g that you don't really need to worry about proportioning with the gt brakes. If you add the larger gt rotorsthe bias will be shifted slightly toward the front so your balance shouldn't be a problem. On the other hand the gt brakes are really not going to make that much of difference to the v6. Replacing the tires will have more of an impact on braking.
If you really want to improve braking, you need really sticky tires, and some aftermarket brakes, with bigger calipers and rotors. Be careful how big you go because you may need bigger wheels to clear the brakes.
