hmm, maybe I should do a quick primer on how brakes and brake proportioning work . . .
The first and foremost thing to remember, is that maximum braking is always limited by how much grip on the road surface the tire has. A tire's grip is controlled by two main factors, the coefficient of friction, and the amount of vertical load on the tire, or basically, how much weight that tire is supporting.
Changing the coefficient of friction can only be done by changing the tire to something different, or driving on a different surface.
Changing the load here is what our discussion is about.
A stock contour is a typical fwd car where the vast majority of the weight is supported by the front tires, even while the car is just sitting there. Because I don't recall the contour #'s offhand, i'll use 70/30 as an example. So, 70% of the vehicle weight is on the front tires, and 30% is on the rear. This means, in the best case situation, the front brakes still can do more than TWICE what the rears can do, regardless of what hardware is being used. Then, when you consider weight transfer during braking the numbers get worse. The actual nose dive of the car isn't a big deal during braking, its the transfer of weight from the rear tires to the front that reduces the effectiveness of the rear brakes. And you should note that, while they are related together, the weight transfer still happens even if you make changes to limit nose dive under braking.
Depending on the initial wieght distribution, and the way the car is set up, you can get so far as to basically remove all load on the rear tires. I've seen photos of even cars like Porsche 911's where the rear tires have actually come off the ground because the weight transfer was so severe (that case was a rear heavy car to start with, but the deceleration rate was high enough to cause the "stoppie").
So it all comes down to what load you have on the rear of the car as to how effective the rear brakes are. If you increase the load at the back of the car, the rear brakes will do more before reaching lockup.
Now for proportioning. For a car that is safe for the public to drive, you need to have a brake system that will never have the rears lock up before the front. If you lockup the rear tires and not the fronts while going through a turn under braking, you will spin the car. To do that, brake designers design in some system to limit the brake torque generated at the rear tires. There are many different methods of setting this bias to the front tires, and some vehicles use several methods, and some use only one . . .
One method is pure hardware sizing. Where the rear brakes are just smaller in general, and capable of less brake torque. This method is pretty common, especially when used in conjunction w/ other methods.
Another is hydraulic system sizing. This is where the circuit controlling the rear brakes is smaller, and provides less pressure and less fluid volume than the circuit for the fronts. Don't confuse this w/ prop valves.
And the final, and most common (in conjunction w/ the hardware sizing) is the use of a proportioning system, that limits fluid pressure to the rears, relative to the front brake fluid pressure. There are hundreds of variations on this, from the simple line restriction like the aftermarket prop valves, to a complex electro-hydraulic unit (the ABS module used for EBD).
An additional prop valve for the rear brakes will never increase braking capability, it will only limit.
Also note that, most if not all fwd vehicles are split into diagonal brake circuits, rather than front/rear like most rwd vehicles.
