I just got a speedlite with a built-in radio receiver this evening for the A99 II. I was excited to try it and see how much power I could get out of it at higher shutter speeds. To my dismay, however, at speeds above 1/1000 there were considerable dark lines across the image. They basically looked like very faint scan_lines. Well, turns out the culprit was the Electronic First Shutter Curtain. Not exactly sure about the mechanics of it but it seems like a similar effect to when you photograph under fluorescent lighting. The electronic shutter thingy synchronizes to the light frequency of the flash (or something idk I'm not an engineer). So if you turn that feature off, you won't get those scan lines at high shutter speeds with HSS. NOTE: It also seems to affect certain lenses at high shutter speeds as well. When using older Minolta lenses, it doesn't cause those weird lines but it causes the exposure to be inconsistent and darker in some frames than it should be. There might be other circumstances in which it might be a problem but these are what I've encountered so far.