Have you read these yet?
https://forums.hybridz.org/topic/38592-brake-balance-faq/
https://forums.hybridz.org/topic/38499-brake-upgrade-faq/
Well from personal experience I have done this set up and the front and rear brakes can not be balanced even with an adjustable proportioning valve. They work okay but this system is overly front biased resulting in the car pitching forward because the front brakes are overloaded and the rears aren't doing enough work.
A braking system has to be engineered for the car, I think if you put the stock brakes back into perfect working order and get a set of sticky tires you would be surprised well the car stops. That or go all the way and install one of the Wilwood brake kits offered by TTT, ArizonaZcar or Silvermine.