Results and performances in friendlies can be discounted.
I have no idea but I'm sure someone on here will have a points per competitive game for Vogts, Smith, McLeish, Burley, Levein and Strachan.
The problem is the quality of players available to the manager and not the manager himself. There is a lot of greetin about the squad selection but those being left out really would only make a marginal difference.