No lag in Bright compared to Game mode HDR? Because that would be fantastic news. I don't have a HDR device for my B6 until November with the One X, but i did have my friends over two weeks ago and we tried the One S and Pro with some games and what we mostly noticed was how ridiculously dim/dark HDR game mode was. It was as if the Xbox One dimmer had been enabled, that dim. HDR Bright and Vivid looked fine. But we didn't test input lag.
So would you say games like Gears 4, CoD, BF etc are playable fine with HDR Bright? I'm still hoping LG simply fixes HDR Game mode though. As for muddy, we didn't really notice it looking muddy, but definitely way way too dim/dark.
As for retention that could result into burn-in....
Last weekend, monday and tuesday i've been playing quite a bit of Diablo 3 during the free weekend, plenty of static HUD elements and after several hours i didn't even see any retention of it. Great news of course. So it really is only happening when very bright things are being displayed. As said before, leaving the Xbox menu, Youtube. Crunchyroll app on for only a few seconds i'll be seeing the retention very soon. I guess the pictures in those apps are overly bright then or something?