Brake guru HELP!

They all make the same line pressure, as much as 900 psi If you stomp it. Larger bore move a larger volume of fluid which helps where large bore calipers and wheel cylinders are supplied.
What size rear brakes do you have and are they working properly ? They should lock down with enough pedal pressure.
Scarebird uses Chevy Cailier/Celebrity brake parts that would stop one of those.
They were meant to stop a much heavier vehicle. Scarebird wouldn't recommend them for performance use. A more agressive brake linning might help.
Yes the OEM Kelsey-Hayes brakes used the same diameter rotor. The difference is in braking surface. That 4 piston caliper and pad grabbed about a third of the rotor diameter.