If I recall the FW* drivers are relatively simple hybrid linear / direct-drive FET arrangements, ala most lights running the Anduril UI. For the low range, linear regulator(s) provide variable current to the LED(s). For the high range, a FET directly connects the cell to the LED(s), PWM'ed at high speeds to vary output.
There are more efficient buck- and buck-boost drivers for numerous FW* models although I believe they still require li-ion input voltages (3.0V - 4.2V).
dang maybe someday such a sweet light but i like redundancy . for now my fenix 18r is decent at least cr123s are affordable online in 50 packs but i rather have a battery i can get localy at every store in the world aka the AA