I think this works because the AB800 was able to expose the entire frame since the sensor was exposed fully for the duration of the flash. Its also possible that 1/320th only works under certain situations so Sony decided to stick with 1/250th which is the mechanical shutter speed (playing it safe). But, some cameras are able to achieve high sync speeds, particularly D70 and G1, so I guess it depends on how the sensor is scanned at high shutter speed.
You can get more info here http://strobist.blogspot.com/2007/05/hacking-your-cameras-sync-speed-pt-1.html
: There are a couple of reasons why a faster shutter speed is important. You are correct that aperture does control flash exposure, but in a situation where the ambient exposure is say 1/1000th, syncing at 1/250th will cause a significant amount of ambient light to be within the exposure and in some cases may result in overexposure. This typically happens when shooting outdoors in bright sunlight. You can walk up the power of the flash and shoot at something like f22, but then your'e dof is huge at that point. Also, some action scenes will still show significant motion blur at 1/250th, for example sprint and car races. So, in order to freeze motion and get sharp results, a higher shutter speed is essential (or a very short t1 duration, providing the ambient exposure is "correct" at 1/250th).
Hope that makes sense.