Just looking at Brad Ausmus’ apparent effect on pitchers.
Roy Oswalt in his career: 3.21 ERA, .687 OPS
Roy Oswalt with Ausmus catching: 3.04 ERA, .685 OPS
Shane Reynolds in his career: 4.09 ERA, .742 OPS
Shane Reynolds with Ausmus cstching: 4.16 ERA, .728 OPS
Mike Hampton in his career: 4.06 ERA, .741 OPS
Mike Hampton with Ausmus catching: 3.36 ERA, .709 OPS
Jeff Weaver in his career: 4.71 ERA, .780 OPS
Jeff Weaver with Ausmus catching: 4.86 ERA, .783 OPS
I picked a few pitchers who threw a lot of their career innings to Ausmus. There doesn’t seem to be much of a trend. Obviously these data should be taken very lightly, as they don’t correct for context and the career numbers cover a lot of territory (for example Mike Hampton was a very different guy during the years he teamed with Ausmus).
If you look at specific teams and years, teams have vastly better numbers with Ausmus as the receiver than with other catchers, but this is because other catchers usually caught the 5th starter.
Anyway, I’m curious how a study like this would play out if done fully and correctly, and how much of a difference in ERA or OPS against would be significant.