There is definately a difference you can feel.

That difference is primarily in the form of having much less brake fade under heavy braking conditions and a little more actual stopping power too.
Now if you go to a good stainless wavy rotor, it will stay even cooler than the standard slotted rotors and work a little better yet.

Putting that into a % form though?
No, too many variables.

The differences will vary depending on various things ranging from your weight, bike weight, tire/wheel weight (rotating mass), how hard you are braking at that particular point, how much you are using the brakes and getting them hot on that section of trail, weather will have an effect on the temperature.

If you are trying to make the most powerful brakes possible, go to a good stainless braded line that will not expand when you squeeze the lever and cause that mushy feel, run good compound brake pads, and also the wavy rotors in the largest size that your frame or fork will allow.

203's are fine with QR as well. 20mm is NOT needed for 203mm rotors.