I could be wrong, but iirc even tho you can boil the water at lower temperature, it still takes the same amount of energy to change phase, so the efficiency gain isnt as spectacular as you might expect.
The energy to change phase, the latent heat of vaporization, does decreases. Enough such that the ambient temperature is able to supply sufficient heat for vaporization (that is, boiling). The latent heat of vaporization is temperature and pressure dependent for most materials.