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.
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.