Thanks for providing the numbers. My math says you increased front brake bias by 37%. That includes 27% more piston area and 8% greater diameter, but I don't know the center of pressure of each caliper, so that is just a guess. Indeed that is not a 2x factor, which is good to know. Hopefully that is within the ABS system's ability to correct, when you reach limit braking. Regardless, you have still added a lot of turn-in understeer when you are driving at the limit, because the front tires will run out of grip sooner than the rears (due to the added braking force). Hopefully you are tuning that out with bars and sticky tires.

I do not know if the brake proportioning valve is replaceable or where it is.

If you are claiming that a 27% increase in front piston area and an 8% increase in rotor diameter amounts to a 37% increase in front brake bias. As in the car going from 50-50 to 87-13, or even 68.5-31.5, I think your method is oversimplified. Please do elaborate on how your arrived at your figure. I worked a lot with some online calculators. But of course you must have the information on the rear brake system in order to arrive at any actual bias numbers. I had to stick with the default values and hold them constant since I had no actual data on the rear.

Here is one of those calculators:

Bias Calculator.

I have messed around even more with this calculator since I originally wrote this post. It looks like using their logic and default values for unknowns the change to the front bias is just under 10% points. A change of ~17%. You have me intrigued and If I find out any more details about rear numbers or how to get the most accurate value of the change to the bias I will post back up. I will also let you guys know if I end up on ice skates at the limit of braking.

I believe, but do not know for certain, that there is no such valve in these cars. Proportioning is handled by the sizing of other components in the system (caliper pistons, rotors).

Edit: added more options of what I think you may mean, trying to get my head around your logic. Also added more figures from my original looking at this.