Interesting post. I had a look at this a while ago and ended up rather confused. I'm just learning but noticed the foreground in some shots were slightly less exposed at 1/320 than at 1/250th. (D7000, wide angle looking into the sun with DS125 strobes on full).
I figured it must be a sync speed issue (strobes are fibre optically triggered if that makes a difference?). Then I stumbled across an article: http://wetpixel.com/...al-explanation/ that shows flash duration curves for different strobes. If I understand this correctly you can see the InonD2000 has a very sharp curve with a quick decay curve. Whereas the YS90DX has a very long duration in comparison - if I do my maths correctly this is getting close to 1/320th.
I couldn't find any info for Ikelite strobes so I emailed them explaining the above and they said that the cause was in fact due to the flash duration. I asked them about DS160's given they are more powerful and they said the full power flash duration on a DS160 was longer than a DS125.
At this stage I just let it go as I was out of my depth. It would be really interesting to hear from someone with technical expertise in this area and see hard data for different strobes at different power levels. Maybe I'm completely off track with this and maybe it has no practical application in the underwater world but I would be curious to know - particularly if the make of strobe influences ability to achieve sync speed.