I finally wrote a spreadsheet to calculate brake proportioning.

Here are my main findings:

Upgrading to a front BBK

It appears that the following BBK setup will have very similar bias to a stock 2003 Sport without ABS, which is the pre-NC stock car with the most rear bias that uses a prop valve.

11" front with 4 wilwood calipers x 1.38" pistons
9.8" (standard 1.8) rear rotors with (2003) 1.375" pistons

The big rear rotors (9.8") with the 1.8 caliper (1.2" pistons) doesn't increase the rear bias as much as the above.

On adjustable prop valves

You can't get the same proper proportioning (over a range of tire/surface grip available) by keeping the rear 1.8 brakes with a front BBK, with just an adjustable prop valve. A prop valve will help fine tune a setup that's close; a prop valve with stock rear brakes and front BBK will enable you to get good rear bias at one set of tires/surface grip, but will require lots of adjustment for optimal braking at other grip conditions. A more optimal setup will be closer to perfect over variations of grip and will require minimal adjustment from a prop valve.

The rear 9.8" rotors with the 2003 calipers will give you too much rear bias and you will really need an adjustable prop valve to bring the rear bias down. This setup however, will give even better proportioning, especially in high grip conditions - i.e. dry with street tires to dry with race tires.

Questions

Now my first Q is -
has anyone experienced fade in the rear or otherwise measured temperatures, with the rear 9.8" rotors?

Is it a bolt-on affair? Are the rear pads the same between a 2003 and a 94~97?

 

1.8 brakes use FMSI D636 rear pad.

Sport brakes use FMSI D891 (I think) rear pad. Bigger, maybe thicker.

Pads follow brackets, not sliders.

Haven't proven it all bolts up on a car yet, but I believe the mixing and matching of which you speak is mechanically compatible.