I'm assuming this will fit the C8 and many S-series lights. 8A is suitable for the SST-40 and SFT40, but will likely be limited by forward voltage with most emitters.
5A is running a 519A, SST-20, etc... fairly hard, while 3A is a pretty average single-emitter light.
I wish they had these available couple of years ago; I moved all my self-assembled lights to 21700 Hosts because cheap buck and boost drivers were only available in 20mm and 22mm sizes.
According to the Convoy BLF thread there is also an 8A buck driver coming in 20mm size. That (or these new 17mm drivers with an adapter) enables more options for the S21A as well.