Just got two 600's and ran a test using wireless transmitting.
Canon 5DmarkII on a tripod. TV. ISO 100
On-camera flash set to No Flash... transmitting only, every other setting at default..
Second flash set to all default as slave. Second flash at 90 degree (side light) of subject.
Both flashes to to HSS
Frame filled with the top of a white table, focussed on one of my kids stuffed animals. (Maybe a chart would have been cooler, but this was still a revealing test).
I shot the exact same image at every shutter speed ranging from 1/30sec to 1/8000 second.
ABSOLUTELY NO SHUTTER SYNC PROBLEM THROUGH THE ENTIRE RANGE.
I uploaded all of my images to Aperture and reviewed. Interestingly, after from 1/250 second to 1/500 second, the image dimmed by about a stop, then brightened back up again and stayed startlingly consistent all the way through 1/8000.
I repeated the test, waiting for each exposure until I heard the slave beep... and the results were identical.
Hopefully this helps someone.