I believe the KL4 is a boost regulator. 6V is lower than the Vf of the typical 5W. Also, if it's a buck, it would never work on an E1e. So I think PK may have overlooked something in his response. So, PK responds to e-mails from CPF'ers /ubbthreads/images/graemlins/grin.gif
The KL3 on a single 123 is running out of regulation, hence it will dim as the cell gets used. Also, it will actually run dimmer than if the LED is run directly from the cell due to voltage drops in a sense resistor and the on resistance of a FET in the buck regulator.
CM