Compromise Agreement to manage out Poor Performers?

We have a situation whereby three managers in the business are failing to deliver in a number of areas. We have started a performance management route with two of them; one of which is already at a written warning stage for a specific failure. We all know that managing someone out through performance management is a time consuming exercise and would like to offer them a compromise agreement to leave.

Is this recommended and what are the potential pitfalls? How do we stand if the employee refuses and continues down the performance management route and is ultimately dismissed - I suspect an ET may not regard the previous CA offer positively?

Thanks