Richard,

It's not a matter of a salt system working efficiently at lower cya levels. Cya levels have nothing to do with efficiency. It has to do with retaining the chlorine in the water. Try the same with a bleach pool and low cya. The chlorine is quickly consumed by UV. Same thing with the chlorine produced by a salt system. UV.
We know, from trial and error more than anything, that the 60 -80 ppm range works. Since ORP controllers are affected by cya, 30 - 50 ppm is recommended.
But cya at lower than 25 ppm? You will drastically see an increase consumption of chlorine, salt generator or bleach, it doesn't matter the source.