Are you upgrading the front brake diameter? The issue as I see it is that the discs on the back are not too much smaller than the brakes on the front so as they have less weight on the rear are prone to locking earlier. In the event that you upgrade the front disc diameter, and increase the stopping force there, I would assume that the bias would move forward even with the std proportioning valves? I am no expert, but just putting my idea forward.