Great review about Nissin i40 flash for Fujifilm X cameras. John shares his experience with flash, tested it in many situations (included TTL). Reviews also includes info how to enable HSS with current Fuji firmware.
Officially the Fujifilm X series cameras don’t support HSS with the Nissin i40 at this time. According to Nissin, they are waiting for a firmware update from Fujifilm that will enable proper support for HSS. This was the reason the i40 for the Fujifilm X series cameras was originally delayed. But since being released, plenty of users (photomadd, wimarys) are reporting that HSS with the i40 on select X series cameras can be activated but this is not TTL.
To activate it you need do turn the flash to manual mode and then press the “TEST” button (NOT the ON/OFF button) for 3 seconds until the small dial’s LED starts blinking slowly. It works with the X-T1 all the way up to 1/4000sec as long as you haven’t set the Electronic shutter option. I hear from others that it also works with the X-Pro1 and X-E2.