I haven't got a chance to make a video like this but it is planned !
the brightness isn't affected per say by the conversion, as the amount of current you can send the LED depends essentially on your battery.
if you had a setup with a 2x18650 cell 2400 mAh (let's say 3A discharge rate), this max payload doesn't change when you go to a single cell (it's still a 18650 battery, 2400 mAh), so the brightness is unchanged.
However, using a single cell places you much closer to the LED Vf (or "threshold"), therefore you'll have a reduced runtime, as the provided voltage by the battery pack is lower.