After peeking around for quite a while, I stumbled upon this thread - https://misterfpga.org/viewtopic.php?t=2514 which mentioned an adapter that properly processes full range RGB. I ordered one, it arrived the other day, I tried it with my PS4 and confirmed it indeed does!
I guess there is a different board revision which uses a Lontium chipset (see the 2nd page of this thread - https://misterfpga.org/viewtopic.php?p=13379#p13379 )... and I believe Lontium chips do the weird thing (not that weird, as full range RGB over HDMI isn't common for a lot of TVs and gear) where they expect limited range on the input and adjust the image accordingly. But mine is one of the units that says 'HDMI' on top of it, which also properly processed 0-255. Looks like ones that say 'HDMI' on them are the one to look out for, and not the EnjoyGadgets ones.
I believe Fudoh has also said the StarTech HDMI to VGA/YPbPr also properly processes full range? It seems like ones that output both signals seem to be the way to go. Atlona has one that appears to be the same exact thing as the StarTech, and can consistently be found for a lower price. There is also a SIIG one that I have yet to try, but it looks promising. It had a DVI-I input, too, as well as digital audio out. Looks nice! The EDID stuff may prove to be useful on these SIIG units. These, too, can be found for cheap on eBay.
Atlona
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SIIG
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I'm very pleased to finally have one that doesn't fuck around with the signal. It's not the most drastic, noticeable difference... but it's noticeable to me and gives some sort of weird, nerdy peace of mind.
If more people can chime in with units they have which pass a proper 0-255 through, it would be great to compile a list! I've been seeing more and more people trying to find these things as more and more people realize MiSTER direct video is, technically, nicer-looking than the I/O boards (for the time being). As well as compatible with interlaced, which I believe the I/O board isn't (correct me if I'm wrong!)