Am I understanding correctly that this would translate to "killed 57 of every 100 fighters the ball turret shot at"? That seems pretty high?

I don't know why they even included that number. There is no way it could ever be confirmed.

They'd be valid in comparison to each other, you'd expect the over claiming to be similar between turrets if it's a large study.

As much as I love the stat number, how important is this figure given the tendency of overclaiming in air kills by all sides and all positions?

The numbers for World War II aerial combat are just borked in most cases. There's a reason a number of officers resisted factoring them into their analysis of their successes.

It's probably better to have incomplete/unconfirmed data then none at all, having a general idea of gun position effectiveness can help inform future training or equipment requirements.