Yeah these 3090 with their 24gb (lol) vram are a complete nightmare to cool, the 3080's as well to some extent but not as bad as the 3090.
My way of thinking for my 4x 3090 is fully water-cooled or don't bother.. 3080's are a bit better on air but not massively.
All you have to do is replace thermal pads with gelid extreme... fixed my memory heat problems on 3090.
How do you replace the thermal pads with Gelid? There is quite the gap back there to fill with paste. I think it's 1mm, or 2mm depending on where the pad is. Also, I am not sure if the problem VRAM are the ones on the back under the plate.
I just realized you were referring to the Gelid GC pads, and not the GP paste. How many did you need in total, and what thickness? I am sure it won't be the same as my Gigabyte Gaming OC 3090 but it would at least give me an idea.