Need to check on it, but I think nothing changes the brake bias unless you install a proportioning valve.
. Let's start at the beginning. Braking Bias is determined by the difference between the front and rear braking power of the car. Brake torque and the grip of the tires are the two biggest factors determining braking performance.
Brake torque = line pressure x caliper piston arear x effective rotor radius x pad coefficient of friction.
Looking at the above equation the two easiest things we can do to change the braking bias front front to rear is to change the size of either the piston area or effective rotor radius. The H6 upgrade on the rear brakes of the impreza changes the latter, providing about a 10% shift in bias to the rear. The 4 pots do the opposite, decreasing the brake torque in the front.
As I'm being quoted above I thought I'd give the chance to explain myself. I didn't mean to say that doing the two together would be dangerous. However if you put both 4 pots in the front and the H6 rotors in the area, you have changed the braking bias quite significantly from the stock setup. These two could probably be used together quite successfully however you might have to be a little careful about pad choices.
Remember that neither the 4 pots nor the H6 rotors will significantly shorten your stopping distances. Go out with a stock WRX and slam on the brakes and the antilocks will engage. That shows you that you have sufficient brake torque to lock the wheels, and the reason you don't stop quicker is due to a lack of grip. Even with my stickier Yokohama tires in 225 width sizes full brake pedal at any speed would still result in antilock engangement.
So what we are really talking about is changing the balance. In my opinion, and take it for what it's worth, doing both 4 pots and H6 rotors would provide more rear bias than you would ever want on a street driven car.
Now, continue to keep discussing me behind my back
Oh, and on a side note, my cars all back together