So the cool thing about the 219C, which we do have data for is the forward voltage curve. It only takes about 3.4V to run it at 4.5A. That’s about 70% of the capacity of a Sanyo UR18650FM under a 5A load.
Yes, a current-controlled buck converter is going to perform better in low modes. The driver required is larger and more complicated, with a bulky inductor on it. Lots of manufacturers and more than a few DIY lights use drivers like that, but it’s hard to find one that fits nicely in a Convoy S-series host. Being able to fit inside the host you’re using is an important characteristic for drivers.
A boost, or buck-boost driver would be ideal. It can keep the output regulated until the battery hits whatever safety cutoff voltage the firmware author has chosen. Single-cell 1.2/1.5V lights use them, as do Li-ion lights from Zebralight and Armytek. I’m not aware of any boost drivers available for higher-powered Li-ion flashlights, though some with the necessary current and voltage ratings do exist in the laser community. Those appear to be based on Ti 258x chips, and like buck drivers, do need some diodes and an inductor.