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.

Well, the efficiency at 2*(3.5 ... 4.2) would be interesting too

This adapter was in my N900 box, that's for sure.

And I tend to prefer it over its own charger because that one makes a very awkward hissing noise when plugged in.

My first N900 was one of the first direct Nokia batch to the UK - it had the charger adapter.

I got a second N900 from Vodafone this week, which also had the adapter in the box.

The first charger adapter died after a couple of days charging the phone in the car for longish (4-5 hour) periods each day.

You should get that replaced under warranty.

Good idea.