I have my D700 set to 1/320th. I did so because, like you, I thought it was a good way to get the most out of the camera. I did not fully appreciate the slight loss of illumination issue.
Anyway, your post made me curious so I decided to do a little adhoc testing. Unfortunately, it is heavily overcast here today and likely to be so for the next few days. With the sky acting like a giant softbox, I was not able to force the camera into high speed synch.
As a fall back, I had a look through some of Joe McNally's books to see what he has to say. Joe seems to have his camera's set to 1/250th and is silent on the use of 1/320th.
Finally, I re-read the manual. As you already know, that was of no help at all.
I would also like to hear what others think about this. Have you considered reposting in the flash forum?