I just ran some tests with one Cree XM-L T5 LED. I'll do some more with 2+ LEDs in series, as the data sheet suggests that'll be more efficient:
200mA output
3.7Vin = 89.6% efficient (2.69Vout)
8.0Vin = 76.6% (2.73Vout)
10.4Vin = 73.7% (2.75Vout)
1000mA output
4.1Vin = 72.2% (2.73Vout)
8.0Vin = 69.4% (2.85Vout)
10.4Vin = 67.2% (2.86Vout)