Malkoff Devices has upgraded their M91 Dropin for Surefire Lights (and others). It now features a new emitter, the Cree XP-L, which results in a tighter, farther throwing beam. The output is approximately 825 measured out-the-front lumens at the moment the light is turned on with fresh batteries and 700 lumens after warm up for 1 hour (on 3 CR123 batteries). After the regulated runtime, the light output gradually decreases over a long period of time.
The M91B is designed for an input voltage range of 5.5-12 volts which gives you some flexibility in terms of battery choices. Check out the new M91B at Malkoff Devices.