The LED is an XP-L High
Density, a 3 volt LED
The driver malkoff uses is a buck driver that only pumps up to 3v to the LED. This allows multiple batteries to be used.
so at 3 volts pushed to the LED by the driver, the LED is pulling ~2.5amps without any help from the driver, which is 7.5 watts
at 2.5 amps, the LED is outputting around 200% of it's lumens when at 1amp. 200% is around 920 lumens.
When at 1 amp the LED is putting out around ~460 lumens
All that is what's happening on the LED side of the driver.
Here is what's happening on the battery side of the driver:
Malkoff's top secret driver at 9 volts is pulling 1.4 amps, which is 12.6 watts.
Amps = Watts / Volts
So, 12.6 watts from 6 volts = 2.1 amps that only increases during use. So like Thermal said, 2.5 is around where you should be with steady use from a 6v setup.
the LED only needs 3v @ 3 amps (9 watts) to reach the 1,000 lumens that Malkoff advertises (per the graph above).
so that shows you how much "power" is lost between the batteries and the LED.