Letting Lee Evans and Marshawn go would be the kind of classic moves the Patriots would do. If it were the Pats it would be heralded as genius because it would seem those players were expendable because of Jackson and Spiller and Johnson and several other recievers. Everyone knows the Pats are a "have" type team and could afford the players if they wanted. Because it is the Bills, it looks like a pure salary dump by a "have not" team. We will not know on Evans until we see the type of player we get for that draft pick.
As you said, I would really like to understand what a team like the Pats or Falcons look at when they consider a player. Billy Beane was big on the OBP. Somewhere out there, there has to be hardcore statisticians that feed some of these teams numbers. Given the Bills recent draft history, it is clear our system was awful.