Got my RL-600b yesterday, and I tested it out in all configurations possible with my ST-E3-RT, YNE3-RX, and the supplied V2 remote. I am happy to confirm that HSS works all the way up to 1/4000 (6D max) with very little gradient. In most photos, the gradation is not noticeable, and only testing against a plain wall can I see that the top of the frame is a little bit darker than the bottom, by about 1/2 stop. Not too bad for a $400 strobe!
I tried the following 3 setups:
1. ST-E3-RT on 6D hot shoe, YNE3-RX connected to RL-600b using PC-sync-to-1/8" mini cable
2. ST-E3-RT on 6D hot shoe, V2 Remote mounted on YNE3-RX hot shoe, no other connections.
3. V2 Remote on 6D hot shoe. No ST-E3-RT or YNE3-RX being used at all.
Setup 1 and 2 both work great with HSS mode, seemingly identically. Setup 3 does not work at all. At 1/180 (6D x-sync) the shots look fine, but at 1/200 there is a significant black bar across the bottom of the frame. By 1/800 almost the whole frame is black.
Setup 1 has the advantage of the proven-reliable, long-distance RF connection from the ST-E3-RT to the YNE3-RX to trigger the Rovelight. I'll probably use it like this most of the time. I can keep the V2 remote in my pocket to adjust the RL with, and to test fire only that light if there is the need to meter it individually.
Good to hear and thank you for the update. Will be purchasing one soon with this info.