I can state that the Thunderbolt 3 ->Thunderbolt 2 -> Firewire 800 daisy-chaining solution for tethering to a camera does not work. Camera used was the Phase XF system with a IQ3 back. You will need a Thunderbolt dock (which provides its own power) in between. Not an elegant solution, and not good for on location-work.
The daisy-chaining does work for FW hard drives that has its own power though.
Apple seriously needs to create a Thunderbolt 3 -> Firewire 800 adapter considering they pushed FW in the first place, and are now pushing Thunderbolt 3/USB-C so strongly.