Honestly...
If you look at ERA breakdowns between Salty and Shopp:
Salty: 4.33 ERA
SHOP: 3.80 ERA
That is a difference of .53 runs a game, which obviously is a huge difference.
Let's look at last season:
Salty: 4.63 ERA
Varitek: 3.56
Now obviously Salty caught a majority of innings, and it helps Varitek that he mostly caught Josh Beckett, who was leading the leauge in ERA for a lot of the season.
I do think it is a valid point that the team pitches worse when Salty is in the game, especially the starting pitchers.
http://www.baseball-reference.com/players/s/saltaja01-field.shtml