[chrismiller]

Is there a valid reason for 1/2000th sync speed, when there are flashes that offer high speed flash modes for exposure times shorter than the max. sync speed?

Flash power is much reduced. It essentially has to do a series of flashes as the shutter slit moves down to illuminate the whole sensor. 

 

So when you go from 1/250 -> 1/1000 you are essentially getting a flash 4 times the power where as with HSS the flash power is reduced as you increase shutter speed.

 

Loads on strobist.com, but a taste here: http://strobist.blogspot.co.uk/2011/12/q...flash.html