Why do you want rear disc brakes?
If you are comfortable with the amount of foot pressure that the front DBs require now, un-assisted, then there is no good reason to add a booster. And if you are happy with the amount of pedal pressure it takes to affect stops, then there is no good reason to change the M/C.
But you do need to remove the Residual valve from the rear line, at the M/C, which the rear DBs do not need or want. It has to go. The Proportioning valve may need to be reworked or replaced..
The rear brakes usually do less than 15 to 20 per cent of the stopping work. Just switching to rear DBs may not slow your car down any faster.
If you have lots of rubber in the back, you can increase the amount of work the rear drums do, by increasing the line pressure to the rear. If there is still room for improvement in the rear, there may be upsized drums available. If you are running skinnies up front, then I can see more rear brake desireable. But you will have to be very careful not to overbrake the rear, cuz rear-wheel lock-up usually leads to spin-outs.
I have found with 295z in the rear, and 235s in the front, I can run rear line pressure equal to front, with no proportioning at all to the drums. I have KH 4-pistons up front and 10 x 2.5s at the rear.I am currently running 15/16 w/cs back there IIRC.
