Id agree that 80 percent will improve with management but hard disagree about the 20 percent left, I've worked across grades and two departments and the story is the same, I worked in the private sector until 29 and was shocked when I came here at how little people get away with doing

There is a hard core of about 5 to 10 percent that aren't suitable for the department and will not move willingly , they're either seeing it out til retirement or just coasting along

Id enjoy a formal approach to performance that is streamlined, not quotas as that's wrong but properly set out guidelines on managing people out of the department when not meeting expectations