Not too bad, I took it slowly and it was a couple of hours last night, and mem temps are >10C lower at a higher load than originally tested. Peaking at 90C at the moment, was hitting 100C easy yesterday morning.
I got the iFixit Mako (from rainforest) set which has everything but the spudger in it so you'll need a thin plastic object (toothpick or something?) to pry off the X piece. I actually used some gaffer tape to pull it over, then lever up with tweezers. Tape is useful later to secure the cables too.
The hardest part was some $%@ at the factory had mixed up the tiny screw head covers and one was totally jammed in as the wrong shape. Had to use a scalpel blade to slowly lever that out, some very fine scratching on the edge but can't be seen at arms length. Alcohol & better thermal paste are the only other items to get.
I got the same pads as in the guide, but if you can find higher conductivity ones, you might see bigger temp drops. I suspect much further will require active cooling of the upper surface of the GPU as that's just passive cooling of the backplate.
One thing I think the video skipped was the ribbon cable connectors have a flip up fastner (the black bit on the mounting). Says it when putting back together but maybe I missed that in the teardown part. And the rubber grommets will fall out somewhere, so put to one side immediately.
https://www.reddit.com/r/nvidia/comments/mmzri2/3090_fe_thermal_pad_mod_cutting_template/
https://www.youtube.com/watch?v=G3260LR2JzQ
Use a knife to cut the pads, scalpel is best.