Hello degarb.

For highest efficiency, you want an emitter like the XP-L2 in the highest bin possible and a buck driver set to 500mA max, or even a triple setup for maximum efficiency.

Since 700mA is a low current, you will not have a problem with a driver like this:
http://kaidomain.com/Flashlight-DIY-and-Tools/Circuit-Board-Driver-Board/BD39-17mm-2_2A-3V-9V-1-cell-or-2-cells-2-Groups-4-to-6-Mode-Buck-Driver-Circuit-Board

It’s an a constant current buck driver, and you can easily set it to 500mA operation with a simple resistor change.

Actually, just ask Kaidomain if they could set the max current to be 500mA. The lower modes will also be lower because of this.

Finally, get a P60 drop in like this, and ask Kaidomain to change the Nichia 219Cs 4000k to XP-L2 5000k, and the driver to 700mA operation if possible:
http://kaidomain.com/Flashlight-DIY-and-Tools/led-drop-in/KDLITKER-P6-LED-Drop-in/KDLITKER-P6-TRI-Nichia-219C-Neutral-White-4000K-High-CRI92-1000-Lumens-3V-9V-P60-Drop-in

With this, you will get the absolute highest efficiency possible, and with a 3500mAh battery, or even 2, you will have some crazy runtimes even on max.