I see the usefulness of maximum flash sync when using balanced fill flash outdoors but shutter speed might not have the effect with flash photography one would expect.
a couple quick examples, please excuse the dirty fan..
This first one is a 1/10th of a second exposure f/8 ISO 100
The fan is set to low and blades are all but invisible to the human eye.
This is a tiny mid-air drip from my bathroom faucet, 1/30th of a second, f/8 ISO 100. The only reason the shutter speed is higher here is I took it first before deciding to go even lower to take the fan shot.
edit: I decided to do a few more tests, this time with Auto FP High-speed sync flash mode which enables the flash to function at lower output for an extended time so it is on continuously (through the entire exposure) during each shot at high shutter speeds.. I would say the regular flash results are roughly equivalent to 1/5000th of a second action stopping power although I didn't test that high. (these results would be effectively the same given a bright enough natural light source without the flash)
Auto FP 1/500th of a second, f/8 ISO 100
Auto FP 1/4000th of a second, f/8 ISO 1600