Single Status Update
So right now on Retro we have an ongoing project called the "Sonic Adventure DX '99 edition", the goal is to restore SADX into what it once was the original Dreamcast version of SA1 in terms of looks.
Allot of research and progress is being done towards this goal but one of Retro's members "PkR" found out an interesting tidbit about the graphical regressions in SADX were handled by Sonic Team, take a read:
I think I'm getting closer to understanding the motives behind all those horrible graphical regressions in SADX. In general with ported Dreamcast assets I'm running into a lot of transparency issues, such as alpha channels not being handled properly, or objects becoming invisible behind transparent walls. Unfortunately it seems to be an engine limitation of sorts, which apparently was so difficult to fix that the assets had to be remade with this problem in mind. For example, the entire Twinkle Park walkway with glass walls was a single object in the Dreamcast version, but in SADX it was split into several models - large tube, small tube, middle frame and two reflections. Or the crosswalk, which was a solid rectangle that used a texture with an alpha channel on the Dreamcast, while in SADX the object was made into a zebra stripes shape to avoid alpha channel problems.
The reason I thought these changes were made due to an engine limitation rather than aesthetic decisions or whatever is that all of the above problems happen on the GC Preview Prototype in the areas that are still using Dreamcast assets. The Dreamcast GPU was able to do alpha sorting natively so the game code didn't have to take care of it, but this aspect couldn't carry over to other systems where the game was ported. It appears that, rather than fixing the alpha sorting issue, they decided to make all kinds of workarounds or just hide the problem by disabling transparency/removing objects altogether. Pretty much any area in the game that looks noticeably different from DC was most likely made to look that way because there was a transparency issue with original assets. In fact you can still find the problematic areas in the final version - for example, in the beginning of Hot Shelter there are green fish tanks; on DC you're able to see the animated waves if you look high enough, in SADX they just get cut off (but they're actually still there). I guess the reason they didn't bother to fix that was because you wouldn't normally look there anyway (or you wouldn't know what's supposed to be there if you haven't played the DC version).
The transparency issue is a huge obstacle to restoring Dreamcast assets in SADX because the glitchy parts will have to be remade in a way that doesn't make the game look worse. I've already reconstructed the Twinkle Park walkway in Station Square that looks somewhat close to the original, but there are many more areas like that in other levels. Some levels, like Speed Highway or Red Mountain, seem to work pretty well, while other levels, like Hot Shelter, are totally broken. It might be possible to recreate the problematic areas with a negligible loss of quality with some effort, though.
It's so cool that this is being worked on - it also explains something i had casually wondered about with Shenmue II's xbox port having some different textures and objects. Makes you wonder that this, or something like it, is the reason why those games haven't been remastered or ported since release.