Flutterby, Lisbeth
I think it will still be about 50%.
The specs for your flash (600EX II-RT ?) give:
Effective flash range with EF 50mm f/1.4 lens at ISO 100
Normal Flash: Approx. 0.5-27.8m
High-speed Sync (at 1/250 sec. shutter speed): Approx. 0.5-14.7m
Thinking aloud .... the reduction in power for high-speed sync is because the flash needs to strobe to provide a continuous light while the curtains move across the sensor. The speed at which the curtains move is constant - it is the gap between them which determines the shutter speed. Therefore strobe time for the flash will be the same irrespective of the shutter speed and the reduction in power will be the same.
Does this make sense?