May I see your 440 Hz CRU screenshot?
Pretty much any common values can get to 400-500hz, but nothing I change seems to matter either
I don't think this monitor behaves in a very obvious way about the way it glitches. I have tried lots of different resolutions and different horizontal totals/back porch values over the months (even though I didn't do it quite so...scientifically... unless that is what you call trial and error ), and it does not change the way the monitor glitches at all. After 263-268hz it feels like the scaler simply can't handle it anymore.
I have mostly been using nvidia because that way I can try different values faster, and also because i can create a resolution with an aspect ratio that is independent of the actual active pixels. The highest and more stable overclocks I have managed were using 1080 as the vertical active pixels value (the monitor seems to prefer the native value for vertical), so to get a playable aspect ratio with the highest refresh rate I need to use a "fake" resolution through nvidia. I'm not quite sure what is going on through nvidia's driver when making a resolution this way, but it doesn't seem as laggy as using "GPU scaling" through the panel. If you think CRU could be more appropriate for trying this I could use it for the sake of testing though, perhaps there is a lower resolution with the correct aspect ratio that could work.
The only mode that works properly with overclocking is the "fulllscreen" scaling option, so it is important that the key you press on the S-Switch is set to it. Some lower resolutions can work on aspect and 1:1 sometimes, but overall it is really glitchy. Depending on how exactly you get into the mode, it behaves differently, and the scaling mode and the resolution itself you were using prior to switching also seems to matter. Sometimes they "work" but are actually at 60hz or 120hz (pretty obvious judging from the mouse movement), even if the OSD says otherwise. I think the monitor is programmed to handle some of the default resolutions different from the others. This monitor is technically the revision of the original xl2420t and features a 144hz mode by default, but the scaling options only work up to 120hz (it will go out of range if you try to do video scaling on anything higher than 120hz). I always thought this was odd... Perhaps they knew the scaler was the weak link of the monitor or that it had some firmware bugs and made it so it wasn't possible to use it on 144hz to avoid issues.
Also I took some slightly better pictures in case you are interested
500hz is all windows will allow me to try