Oh a car without ABS, I would not do this modification. My valve is completely bypassed, so I have full fluid flow to the rear brakes at all times. The increase in braking power was substantial - I can trigger the ABS at all four wheels now, something that was never possible before. My autocross times show that it was a definite improvement.
I wonder if the larger pad in the rear of the early cars will cause this effect to be exacerbated. I don't have any problems with ABS triggering on normal stops, and I don't think John (DeaconBlue) did either. The early pads for the cars with vented rotors are quite a bit bigger.
Bottom line: I love it, but it's probably not for everyone.
John V