If we assume lightbulbs of similar efficiency in terms of lumens per watt, there is nothing to be gained by stepping down to low voltage. The primary xfmr current in the above example works out to 0.17A (ignoring xfmr losses), but if you had a 20W 120V bulb connected directly to the 120V line that too would draw 0.17A, so the meter would still register 20 watts of power being consumed.
As Ian pointed out, you would likely be running less efficiently due to the xfmr and copper losses.
I remember I actually had a hard time explaining a situation like this to somebody a couple of years ago. They had a hallway with two pendant lights, each containing a compact fluorescent, about 18 watts each.
They had bought two low-voltage spot assemblies, each with three 12V bulbs rated at about 50W each.
They were convinced that because they were low voltage it was going to save energy, when in fact they were planning on replacing 36W of fluoro lights with 300W of spots.
I fitted them as that's what they wanted there, but I'm still not sure I convinced them that the low-voltage lights were actually going to cost more to run.
