I have just added an R1 macro flash, and I'm not yet able to get it to sync higher than 1/320 with the built-in flash as Commander. The D300 won't let me select a shutter speed higher than 1/320 if the built-in is popped up.
Per the D300 manual .288 and also the R1 manual it looks like the R1 supports FP sync at > 1/320.
I have a SB600 available but haven't tested with that flash as the commander yet.
Has anyone got this to work with either of the above as commander, or know definitively that it just isn't possible?