There are gains.
If it is set up properly with decent parts, you'll get less rear wheel lockup. They will fade less in heavy usage.
I went from 12" front disc/10" rear drum to rear 10.7 disc. There were no improvements in braking distances but at that time, I hadn't really found the limits of the rear brakes. Servicing was easier with the discs.
Remember, a 4 wheel disc setup uses NO proportioning valve but can use a distribution block. It is the size of the calipers that determine the proportioning. I have 2.6 piston calipers up front and 1.5 piston calipers on the back. You always need the fronts to be bigger since the front does 65-70% of the braking.