Originally Posted by Davenow
Flatirons would know more specifics on the Group-N cars. But I assure you that the brakes are closer to stock than you are thinking.
Group-N cars are basically limited to using any OEM parts from the manufactures world wide production in the same year as the car that they are running. Keep in mind that Rally tires are only available up to 15" so the brakes must fit under a 15" wheel for rally application. So the short story is that the Group-N Subaru Rally Cars use the Subaru 4-Pot/2-Pot calipers.
Also, worth considering is that the idea in a stage rally is to have the average stage speed at around 70 mph or so. This is why the cars run restrictors, etc. And if in testing the cars are able to get up over 100 mph for a long period of time (long straight), it is not unusual for the event organizers to put in chicanes in the straights to drop the average speed for a stage.
Now that being said, there are a number of other differences going on with the Rally car example. A couple of the big ones being no ABS, and braking on a lose surface (dirt, gravel, etc.), which make it a less than ideal comparison to a stock Subaru that is driven on the street.
A better comparison would be of a stock 02 - 05 WRX to an 06 - 07 stock WRX. They both use the exact same front rotor (the rear of the 06 - 07 is slightly larger in diameter, and is vented where the 02 - 05 is solid). So the primary difference is the clamping force from the 4-Pot/2-Pot calipers and the slightly larger diameter rear rotor.
When you are trying to improve your cars braking, there are a couple different ways to go about that. One is to increase the clamping force of the caliper. One is to increase the diameter of the rotor to give the caliper a better mechanical advantage. And the other biggie is to put in pads that have a higher coefficient of friction.
A couple of more subtile options are to increase the cooling to the calipers/brakes to help them handle an increased heat load. And another would be to increase the braking of the rear brakes independant of the fronts to shift the brake bias more to the rear so that you have more even braking front to rear (with a larger diameter rotor for instance as in the H6 brake up-grade, or by putting on the Subaru 2-Por rear brakes for instance. Another option would be to put a more agressive pad, within reason, in the rear compared to the front). That is of course assuming that the OEM system is too heavily biased to the front, which most would agree the WRX was at least from 2002 - 2005.