It all depends on what you want it to do. The amount of current that the driver can handle is going to be directly related to the size and cost of the components. These are things you can’t escape.
For a low current driver, say 3A, you can get a pretty small driver, 17mm for sure, maybe even 15mm or smaller. For a higher power driver, say 6A, 17mm would be a tight fit for sure as seen by the mtnmax with it’s 5.5A limit.
That could be vastly improved on in the feature and firmware but the actual current could not be improved all that much without going to an external coil.
If you did go to a stacked coil inductor setup, then figure you could get a fair bit more current out of the driver, not sure exactly what components options there are but I would guess that a tall 17mm driver would max out around ~8-10A without potting the driver, although even that would be hard as shown by past attempts.
This is why higher voltage is going to become more and more important. Regulating more then 8-10A gets exponentially harder in these small lights. On the other hand regulating a higher voltage setup with lower current is easy.
For example this GT driver would die a very swift death if used in the Q8 with 20+A of current at 1s. You would need something like the Texas buck to handle 20A at 1s and even the Texas buck was only designed to handle 15A. The cost would go from $10 to$25+ to handle that extra current.
On the other hand if we raise the voltage to 4S we only need 5A of current to give us the same output power, this means we can use the much cheaper GT setup and get a more reliable driver as well since it will not run as hot.
As a side bonus we can maintain regulation longer with higher voltage.
This is why I am a big fan of higher voltage, it simply works better for all mathematical, electrical and scientific aspects.
This is also why I am a big fan of the 21700 cell. With the larger cell comes larger drivers, that extra space gives us a lot of wiggle room for driver designs.
