One thing that I noticed is once a team gives a quarterback the big contract, often, but not always his play regresses a bit. In some cases it is a "I got my money so I don't have to work as hard" mentality. In most of the cases it is that the big contract uses up so much cap space that the teams have to cut back on other positions (offensive line, receivers etc)
I have seen many times when a QB, or any other position for that matter gets a huge payday the team does not play as well overall. I think one of the reasons for the Patriots long success with Brady is he has not been one of the top paid quarterbacks despite having by far the best resume. He takes less money so they can build the team around him.