It makes a huge improvement. I decided to keep drum rears and had to add a proportioning valve after the combo valve to reduce pressure on the rears after the swap because the rears would lock first. I have power stop slotted rotors, crown vic 73mm calipers, MM stainless brake caliper sleeves and MM rear shoes. Also have SS lines front and rear, Sn94 MC, 3 to 2 port conversion and 93 Cobra booster. This setup works really well for the street.
My car came with the 86 SVO 73mm Calipers. don't ask me how that happened but when I went to change wheel bearings & seals, thats when I found out. I don't use a prop valve, just the stocker and brake hawk pads, car stops pretty good considering the rear drums (with carbon fiber shoes).
(still wish I had 4 wheel disc's tho)