Original post: https://www.reddit.com/r/Noctua/comments/1t2nnsa/rtx_3080_ti_suprim_x_noctua_mod_done_right/
Thank you all for pointing out the huge gap between the shroud and the tip of the fan and the potential performance loss. I have now fixed that and added a ring that brings the space from 5.5mm to a little less than 1mm (a touch less than the Noctua frame). It is pressure fit and is removable via 2 holes with which an allen key (or any L shaped tool) can used to pull it up.
It improved performance significantly! Now, without any undervolt and stock fan curve, it reaches a max temp of 80°C when stressed, fans being barely audible (huge step up from OEM fans which were really loud). I don't have any noise-testing equipment but it is really quiet. As for the temps, it may have improved it by a couple of degrees, I'm pretty sure with stock fans it reached about 83°C although that was not my main focus.
If you want to do the same, I have linked the files in my original post (with STEP files for you to modify) and if you have any questions, don't hesitate to reach out!
To address some other points:
Why didn't I just deshroud the GPU/make my own shroud: I like the OEM shroud and I want to keep the card itself completely unmodified.
Aesthetics: I think it looks cool, I picked the OG brown fans because Noctua. Plus my GPU is mounted horizontally so I don't even see the bottom of the GPU anyways.
Why not bigger fans: same point as "why no custom shroud" but also because I want the card to keep the same profile, and I certainly do not want to make it bigger, I have other PCIe cards in my machine.
Just make a custom fan curve and undervolt it: Believe me, I tried. No matter what I did, the fans always ended up at 100% and they were loud at slower speeds anyways.