"Improve" in what sense? "Too highly" by what measure? And why do you want to reward mediocrity more highly than excellence?

An exam is partially meant to measure how well the student learned the information. Why would you want to penalize students doing well?

Are you familiar with the story of Procrustes? (0) Who decides what is too good or how much study is too much?

0 https://en.wikipedia.org/wiki/Procrustes

I sort of doubt that. That could eb the case if they were moved to the advanced track where they would be challenged. I don't think lowering their score would be beneficial.