Worked for my humming bird shots. A lot of mine were at 1/200 - 1/400 sec and i was getting sharp wing tips most times.
It really depends on a lot of things. I think the main reason it worked well with hummingbirds because it lit the wing via flash duration and that showed up on the sensor and trumped the slightly blurryness if that makes sense.
Think of it like light painting on a long exposure, but now we are putting a 1/20000th sec flash duration to "paint" or light that moment in time, then the sensor records 1/200th sec but mostly just picks up that sharp well lit 1/20000 sec blast
I like to use flash, but with the bracket, better beamer, flash cable, and flash it really adds a lot of size and weight to an already big setup. Its pretty mandatory to use a monopod then, probably 15+ lbs. using the 500mm f4 of coarse.
Fp or the newer flashes are helpful so when you want to keep shutter speed high and not be limited to the 1/250th flash sync