It 'COULD' make a feelable difference IF you got the right brake balance again.
To work out brake balance... its kinda tedious, but you measure your weight on all 4 corners, make a front:rear ratio, wrk out how weight is transfered to the front during braking and then you can see how much force your front brakes need to deal with, and that tells you how much force you rears have to deal with. If you stick in big brakes on front but not rear, it could actually make you stop slower. But as your talkin bout changing rears from drum to disk.... dont see how you could screw it up that much.
The biggest difference you might see between disk/drums is under constant hard breaking (eg a trackday). If you did it once or twice, doubt youd see any big improvemnt.