This article “Are You Working With Energizers or Rotten Apples?” is interesting. By measuring somebody’s effect on the relationships within a company you can predict their success. You can predict the success of their projects, promotions, customer relations, and follow-on work.
This means that you can’t afford to try to fix non-performers for very long. Or even just leave them in place. Hire slow and fire fast. It sounds cruel, but the rest of the people in your organization deserve it. Do it for them.