One of the cool things about the Nikon D300, D300S, D700 and D7000 is that they can boost the flash sync speed above their normal sync speed of 1/250 to a third of a stop higher to 1/320. The manual (at least on my D300) cautions that using this 1/320 sync speed will cause a reduction in the effective flash output but it is still more efficient than high speed sync (which has a 2-stop penalty). It was always a mystery to me as to how the boosted sync speed works, and what the power loss was.
Neil van Niekerk just posted an article that tests the 1/320 sync speed and compares the result with 1/320 high speed sync, all using manual flash at full power:
I'm amazed that the reduction in effective flash output with the boosted 1/320 sync speed is not as significant as I thought. It's certainly far more efficient than high speed sync.
Neil does note the slight darkening along the edge of the frame and for that reason would prefer not to use 1/320 sync. In my case, I usually use high sync speeds in bright ambient conditions, and the darkened area would normally be illuminated by ambient light anyway. So for my purposes, I would definitely turn on the 1/320 boost option.
On a related note, David Hobby found out that the D7000 can fooled into syncing at 1/400 (up to 1/4 manual power) and 1/500 with some darkening of the frame: http://strobist.blogspot.com/2011/03/nikons-d7000-sync-dial-goes-to-eleven.html (His rationale for going all the way to 1/500 despite darkening is the same rationale I have for turning on the boost option.) I'll test out the 1/400 sync speed on the D300 as well -- it may just work.