3090 FTW3 Ultra here. Installed Hybrid kit a couple days ago. 100% stock thermal pads / paste. I wanted to install it "the way EVGA intended" to get a baseline before mods. And I'm happy with it, mostly. GPU temps dropped around 15-20C overall, so I'm happy with that. I would recommend the hybrid kit. I've used the stock thermal compound but will probably try some other stuff later just to see what kind of difference it makes.
What I'm not happy with is the thermal pads for PWR and MEM, some of them were not properly aligned during manufacturing and I'm almost certain that one of the thermal pads is too thin / not making contact, which would help explain why my PWR and MEM temps are always so high. I'll be diving back in at a later date to correct these.
For now, watching ICX3, MEM chips are by far the hottest, running into the 70C range. Cranking the onboard fan helps a a lot, but I'd still like to see more efficient heatsinks to keep everything thermally balanced before pushing more air on it. Will probably experiment with providing additional backplate cooling too.
Great potential with this card, hoping to see updated XOC BIOS soon and a fix for the fan/RGB controller (fan speed spikes).