Rule: Sync speed is the fastest shutter speed you can use with flash, period.
The 5D's flash sync speed is 1/200. At a recent wedding just before packing up, I tried experimenting with my cam's sync speed by setting it to 1/500.
I then asked our assistant Archie to illuminate one side of his face with a flashlight. Here's the result:
Tech specs:
Canon 5D 1/500 f/4 ISO 800
Then we grabbed Jay and Gracious for one last portrait before we called it a day:
Canon 5D 1/500 f/2.5 ISO 1600
Interested in offcam lighting? Know the rules from sites such as Strobist and FlashFlavor.