Adding a hydra-boost, or any brake booster isn't going to increase the pressure to the calipers. All that a booster does is reduce the amount of pedal effort the driver has to exert.
What is important is the amount of pressure applied against the rotor vs the amount of pedal travel. Also the pressure reading at the bleeder screw is only telling you the hydraulic pressure in the system. What you need to verify is that you have enough pressure being applied to the brake pads themselves & ultimately the rotors. In other words; you may have 1000 psi at the bleeders indicating 1000psi of hydraulic pressure in the system but if the pistons are not moving enough to press against pads to stop the rotors it's not going to matter. So you need to check the actual pressure being squeezed against the rotor by the caliper/pads. They make gauges to do this. Maybe you can rent one from a parts store in your area, Idk. If you can't get the brake pressure(force) tool I mentioned then you can try measuring the temperature of each rotor from both sides immediately after a few hard stops with an infrared temp gun and compare the differences. They can be bought pretty cheap at Sears or Harbor Freight.
I know you have said you had a larger diameter bore M/C on the car & it didn't stop well with that either but I would not rule out the possibility that a 7/8" M/C is too small. What you have to understand is the volume of the master cylinder to the volume of the caliper pistons. A smaller piston M/C has to move a greater distance to move the same amount of fluid against a larger total area of the caliper pistons to create enough force to stop the car. People who convert to manual disc brakes often go to a smaller M/C bore to reduce pedal effort because they don't like the rock hard pedal & higher effort required to stop the car with the larger M/C. And that's fine as long as the smaller M/C can move enough fluid to fill the caliper piston chambers in a relatively short amount of pedal travel.
Now take into consideration that you have 4 pistons in each caliper to move to apply the braking force. That's a lot more area to fill and now it's with a smaller M/C. I think of it like trying to fill a bucket using a smaller bucket then switching to an even smaller bucket. The smaller bucket takes less effort to lift the water but a lot more movement to move the same amount of water.
Also I would re-plumb the lines so that the rear M/C chamber operates the front brakes & the front chamber operates the rear. Also there are mechanical differences in FWD M/Cs from RWD ones beyond bore sizing. There are springs inside on the pistons that are calibrated accordingly. If it was just about bore size then they'd all share a similar part # with the same bore sizes.
