The maximum ISO setting of the camera is not in the manual, I just checked it on the camera using the ISO control. So the flash is the limitation with respect to the highest ISO you can use.
I just looked at my AF280T. The highest ISO setting on it is 800. But I don't think that changes anything electrically. It just shows you your ranges at each f stop. I may burn a test roll and try it at ISO 1600 and see if TTL flash is accurate.