I think you've got it too, lol. The clue was in "Mastering The Nikon D700" by Darrell Young and James Johnson. It states: "You can safely leave your camera set to 1/320s Auto FP or 1/250s Auto FP all the time, since the high-speed sync mode does not kick in until you crank the shutter speed above the maximum setting (1/250 or 1/320). Below those shutter speeds, the flash works in normal mode and does not waste any power by pulsing the output."
I apologise for this breach of copyright, but I hope I have promoted the book rather than plagiarise it.