Probably not much. I was not brave enough to run it long.
With 2x18650, it only drew 0.4A, rising a bit as the voltage dropped. I did a decent charge of the phone in a couple of hours, and the ca-146c did not get overly hot.
I doubt it is much more than 500mA out in either case, so the efficiency of low-V 1.6A would be 50% or worse, and increased resistive losses in the battery & cable. I'll stick to using 7-8V in with 2 cells and keep the magic smoke inside.