Those are tough to come by +Brandon Fritz
. I've worked with a few (large multi-author sites) and seen what I believe are some very solid results.
But there are a number of mitigating factors. Authorship doesn't roll out for all authors all at the same time and you don't actually have visibility into each author's performance unless you ask each author to look at and report their Author Stats data.
I'm personally confident that it does have an appreciable impact but I don't have bulletproof
data to present.