Some data points from that circuit produced this graph:
Relative to the output at 5 mA, the efficiency drops off pretty much linearly:
That’s computed as the ratio of the actual light output to the output at 5 mA scaled by the current ratio.
So putting the screws to the LEDs does produce more light, but not quite as much as you’d expect. Above maybe 40 mA, it’s not worth the effort.
Now we know…