I don't really know how this has been studied,but you're right that there is plenty of anecdotal evidence recently.

Bobbie V provided anecdotal evidence you can screw a team up. He only managed one year in Boston. They add talent and Farrell looks good. Notice that he did not make a Toronto team better.

Manual has a lot of talent from the contending years playing for other teams. That the team is in such a decline that they are giving extended tryouts to jettisoned players like Young should be an indication of the dropoff of talent with the Phillies. Not to mention pitching injuries and old age.

Cleaveland is dramatically improved, look at the talent they added or were lucky to find. Masterson stepped up. Is that the manager or the pitching coach.

I think Farrell got more out Toronto than Gibbons is, and Toronto added a pile of "talent."

I believe that managers can make a huge difference. They can drive their players crazy or keep them upbeat. They can allow them to get sloppy or demand excellence. They can demand excellence while being total jerks or they can make every player proud of what they are accomplishing.

I think there is a lot of evidence to support that.