A major reason modern cars have 4 wheel discs is they are better able to handle ABS inputs. Most of the anecdotal cases mentioned so far center around emergency/panic braking, I assume most know approx 70% of your braking is on the fronts, and discs there are very useful in many situations. And the OP asked I believe mainly about rear disc addition. Furthermore, any braking force generated beyond locking up your tires, in a single panic stop, at any speed, is useless. Remember Tires stop your car. Nobody here is modulating their rear brakes close to lock in a panic stop. Normal thinking is front's should lock up before rears, and they are the ones modulated if the driver has the skills/experience. OP should spend his time/effort/money in places offering much better bang for buck.

edit: with full disclosure I'm currently adding rear discs to a street driven car , many reasons other then performance/braking issues, like rear end change (7.25 to 8.75), BP change, internal drum parking brake, tiny discs ( weight & MOI), car will see some track time, etc