I recently finished a 2 cell 3 watt circuit for the Fenix. It draws 1.5 amps from the cells. The IC I'm using is designed to draw pretty close to the same current from the batteries regardless of there voltage (switch current regulated). At 1.5 Amps draw from the cells, the math for power output for this circuit is 1 to 1 minus about 10-15% conversion loss. This puts the output at about 3.2 watts nominal. I looked at it's output with my O-scope and it meets this expected performance. I tried some experiments and I found that power to the Luxeon is the same (for most intents and purposes) regardless of Vf in the range of 3.2 to 4.2 Volts. If Lithiums are prefered over NIMH's the power will go up. The Lux would be driven at about 3.8 watts max.