The brake modification didn't do anything in previous Gran Turismos.
You can increase braking power of any vehicle by adjusting the brake balance in the tuning menu. For instance, by default the brake balance values are 5 for the front and 5 for the rear. If you increase these values to 10 you will have increased clamping power from the calipers. However, this increases the chance of a lockup, especially if you are not running slicks.
If you do not believe me try doing one of the license test where you are required to stop within the relegation zone. After adjusted the values you will notice that you can drive the car in much deeper.
