Stat geeks are awesome, and very good at what they do. A catcher's impact on a pitcher is a very strong point of interest among them, one they have studied intensely, and they are making great strides in this area. After adjusting for pretty much every factor one could think of, including things like blocking pitches and pitch framing, Varitek earned 57.something more runs in that 4 year span for his pitchers than otherwise would have been expected. Believe it or not.
What's really good about this data is that it gels very well with the what one expect based on reputation.