I ran a fj80 master cylinder with stock 85 small calipers for almost 8 years before getting a dual diaphram booster.  brakes were a bit stiff, but not bad.  the dual diaphram booster will definitley make a world of difference.  i was going to upgrade to the ifs calipers and bigger rotors, but after the booster swap, i'm going to leave it alone.  it stops no problem. with hardly any pressure with one foot, it stops like new car