I just found that the latest firmware for X-T1 / X-T10 can now support 1/250 flash sync.
I've tested with Nissin i40, as well as Cactus RF60 with V6 transceiver.
It didn't work before, but now it does!
I checked the release notes of all previous firmwares, but such update has never been mentioned before.
It seems that Fuji hasn't officially announced such update yet, but the feature is there already.