That is probably not a great combination. The high Vf of your K bin emitter means the Downboy will drop out of regulation very quickly with a single Li-ion cell. Unless you want to use more than one Li-ion cells in series, or more than 2 CR123 cells in series, you would be better off with a Wiz2 driver.
The Downboy can work great with a single Li-ion cell and an H Vf bin emitter, and will run in regulation for most of the cell life if the Downboy current is set for 500-700 mA.
The big advantage of the Downboy is stability in resistored low mode. The Wiz2 and Nexgen drivers usually flicker a bit in low.