Thu 31-Jan-13 01:13 AM | edited Thu 31-Jan-13 01:14 AM by ZoneV
I believe the 1/320 still uses a single burst and not FP; it just optimizes the sync. timing in such a way as to barely eek out a little more shutter speed. You lose illumination, but not as much as you do in FP mode. If you're set to 1/250 FP, the camera will use FP at 1/320th and you'll lose even more light. That's the difference as I understand it.
To learn more about how it works, I suggest reading up on "hypersync"; I believe it's a similar idea.
Many pros who use studio flash units wouldn't want to exceed the maimum sync. speed, because their studio units sync. at much slower speeds (due to their output versus time curve?). That can be a full stop slower than the normal sync. speed in some cases.