Normally, it is cheaper and easier to rejuvenate under-performers than it is to replace them. Marine Drill Instructors, despite their legendary toughness, refuse to give up on any recruit. Demand that everyone act with honor, courage, and commitment. Every organization is unique, hence it will require a different percentage. But every leader and supervisor must display some form of leadership, they cannot simply be bosses and expect the organization to become a great organization.
Managers can get things done, but they have to have leaders and leadership abilities of their own.
It takes leaders to have visions. Once you have your vision, it needs to be framed in general terms and communicated to your team. Just as there are many managers who cannot lead, there are many leaders who cannot manage. And neither is better for a company. Both strip the company of a valuable resource a leader who can get things done or a manager who can lead her employees.