a small mcyl will give you a soft pedal with more travel (same pressure over a smaller area but you need to move it further to move the same amount of fluid), a big one will give a harder hard pedal with less travel. so a larger mcyl will give you a pedal that's harder still.
bleeding wll give you a harder pedal not a softer one, as you're not compressing air.
the GTR does have a vac tank for the clutch.
sounds like the booster might be faulty or disconnected, have you got the directional valve for the vac feed in the right way? brake boosters do develop leaks, i got mine reco'd for $200 and it made a big difference.
if the pads are race type this can sometimes give a similar feel, it's just that you are pushing so hard to get the thing to stop when they're cold that it feels like a hard pedal.
-7 to -21 is a big drop, have you checked for leaks elsewhere?
do you know what was like with other brakes or were they on the car already?