Check if something from Zebralight works for you. Their buck-boost drivers are pretty much the most efficient drivers used in flashlights but they are not available for DIY.

Edit: If you want to DIY I think the LD-A4 from led4power may be a good option since it uses constant current rather than PWM, so the efficiency would be better. However a good buck driver may still be a bit more efficient .

Maximum current may be set from 2A to 3A and the mode groups are:

  • 100%
  • 9% - 100%
  • 16% - 100%
  • 2,5% - 20% - 100%
  • 2% - 10% - 30% - 100%
  • 1,5% - 7% - 18% - 35% - 100%

For example with the 16% mode and 3A max you would get 480mA. With 2.5A and the 20% mode you would get 500mA.