Your question is quite complicated...
I do not think anyone has ever measured the brake line pressure vs. varying input pressures to measure the actual line pressure bias on the 84-87's, but I did do this on an 88 as part of this thread:http://www.fiero.nl/forum/Forum2/HTML/121747.html
Here is a summary of the data and what it shows is that at low pedal input pressures the front brakes start out with about 57% of the line pressure and it increased to 64% once the combination valve has reached it bias limit (very early on). The 88's use the same diameter rotors and same caliper piston diameters, so this table is also the overall brake bias for a stock 88 Fiero.
Caliper Line Pressure Comparison Stock 88:
Front Rear %
Stock Stock Front
Input Air Booster Booster Bias
5 n/a n/a n/a
10 400 300 57%
15 600 400 60%
20 800 500 62%
25 925 600 61%
30 1200 675 64%
35 1225 700 64%
40 1250 700 64%
45 1300 725 64%
50 1325 725 65%
55 1375 775 64%
60 1400 775 64%
As you change the caliper piston areas and the line pressure remains the same, the clamp load on the brake pads increases, increasing the stopping rate.
As you change the rotor diameters (caliper position from wheel center) you change the mechanical advantage of the caliper, which can also increase the stopping rate.
Impact of Caliper Piston Area:
Stock 88 = 48mm = 1,809 sqmm (Centric lists the 84-87 caliper pistons to be 48mm as well)
2.5" Caliper = 3,156 sqmm (75% larger than stock)
2.375" caliper = 2,900 sqmm (60% larger than stock)
2.25" caliper = 2,564 sqmm (42% larger than stock)
As you can see, all of your caliper options for the front GREATLY increase the caliper area and overall clamp load and will significantly alter the brake bias to the front.
Add using these larger calipers on a larger rotor and the change is much more significant.
Impact of the Rotor Diameter (actually, caliper placement further from the wheel center) vs. stock 88 Fiero:
Stock 84-87 rotor: 9.69" (246mm). Estimated to be in the 3.8 to 3.9" range (based off the 88 caliper geometry) or about 11% less caliper leverage than a stock 88.
Stock 88 rotor: 10.43" (265mm). Center of caliper piston to center of wheel: 4.3" = caliper lever arm for braking force.
12" C 4 rotor: 12.01 (305mm). Center of the caliper piston to center of wheel: 5.2" or a 21% increase in the level arm. (this is slightly more than the rotor diameter change as the caliper must sit higher on the larger rotors for proper bridge clearance).
I have never done a LeBaron swap, but the caliper piston will likely be in the 4.8" range or about 14% increase in the lever arm length vs the stock 88.
So if you run the stock 84-87 front brakes and the stock 88 rear brakes... the piston diameters stay the same, so the only change is the impact of the larger rear rotor which would be 11% increase in rear braking leverage.
Using the Lebaron rotors and the 2 1/4" caliper would increase the front braking by 42% due to the caliper size & about 25% due to the increase in rotor diameter. In the rears even if you up-size to the 12" rotors, you only see a 35% increase in braking leverage vs. stock 84-87, so this system will still have a major shift in brake bias to the fronts. To get this system back close to the stock brake bias would require replacing the combination valve with one that would send more equal line pressure to the rear brakes.
[This message has been edited by fieroguru (edited 12-25-2014).]