Yes, higher ISOs can certainly provide an instant power and range boost to our flashes, and save us a lot of battery costs as well! I can't buy one yet, but apart from the obvious landscape / higher DR performance, I'm looking to a K1 in future for what it will offer for flash work indoors ..... for events and in larger rooms, on-camera bounce can become much more versatile if you can happily shoot at ISO 6400 and not have to worry about ugly noise in the dark shadow areas. The extra power and range you'd get gives plenty more potential for getting nice light off higher ceilings, or walls further behind and farther away from the camera position. Something to look forward to.
Ironically, many people commonly link high ISO performing cameras to NOT needing and not using flash ..... I'm the opposite