To answer Stingray’s questions…
Protected cells generally don’t fit in the Emisar lights.
FET-based drivers are essentially direct-drive, so the output on turbo depends on where the battery’s voltage sag curve meets the emitter’s Vf curve. The battery may be at 4.2V, but that value drops as amperage goes up, and it drops as remaining energy goes down. The emitter’s voltage starts at around 3V, but it rises with amps and temperature. Where the two curves meet is what happens in direct drive.
A low-amp cell sags more than a high-amp cell, so the curves will meet at a lower amount of current, and emit less light.
Lower Vf is not always a good thing. In particular, it is strongly recommended that you do not use a 219c emitter with a FET-based driver. The curves meet at such a high amperage that it kills the emitter. This can be avoided by using a very saggy low-current cell, but then it’s typically somewhat bad for the battery instead. It can also be avoided by using several 219c emitters per cell, but even then it has been known to turn emitters into smoke.
The bad-for-the-emitter vs bad-for-the-battery thing is similar to going down a mountain in a car. Riding the brakes wears out the brakes quickly, but generating resistance by putting the engine into a lower gear is hard on the engine. The brakes here are analogous to the LED, while the engine’s gearing is analogous to battery choice. The difference between battery voltage and emitter voltage is the slope of the hill. Does that make sense at all?
I haven’t tried it, but one potential option for safely running a 219c direct drive would be to use a LiFePO4 cell. Instead of going from 4.2V to 2.8V, those are more like 3.6V to 2.8V. It might work well for low-Vf emitters without having to worry about damaging the LED or battery.