"Strong current flow" is informal language, but both it and photoresponse refer to the electrical power that comes out. In theory you would just divide that by the incoming solar flux and get the efficiency. For now it's only in a lab setting, though, so we'll have to see what the practical efficiency will be if this is actually incorporated into a reasonable solar cell.

So yeah, apparently barium titanate solar panels used to be extremely terrible, and now they might become competitive with further research.

It's not a measure of efficiency per se, but efficiency is a limiting factor:

In order for a solar panel to put out 1000x more power, the baseline you're comparing it to must be at most 0.1% efficient, because otherwise the new thing would have greater than 100% efficiency and that isn't possible.

And that's a purely thermodynamic argument. The actual limit for solar efficiency is likely less.