Finally got my 3080 XC3 Ultra and put it into my Sentry 2.0!
First thing I did is undervolting, and I used what Optimum Tech suggested: 1845mhz at 850mv, card is running great.
I played SOTR for about an hour and graphics card temp stabilized at 79°C, with fan speed at 75% (didn't touch fan curve). Card is now running between 270W and 290W. The fan is so much quieter than my CPU fan (Cryorig C7G with Noctua fan). My CPU (3700X) temp averaged at around 70°C with spikes reaching 80°C.
Overall the fan noise is acceptable, and I can't really hear my GPU fan or PSU (Silverstone SX700-PT) fan because my CPU fan is much louder. One reason why it's so loud could be because it's sitting too close to the panel and there's turbulence.
3080 is a beast, running all my favorite games at speed I can't imagine before. I'm using a 21:9 1440P monitor and I can run Borderlands 3 between 110 and 130 fps with every setting cranked all the way up!
My gaming rig is now 99% complete and all I need is a Zen 3 CPU
(and maybe a gen 4 riser cable haha).
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10/29/2020 Update:
I couldn't stand the CPU fan noise anymore and switched back to Noctua L9a with 10mm fan shroud. There's a huge difference in noise, and L9a actually keeps CPU cooler than Cryorig C7G by 2-3 degrees! I also found that my GPU runs fine at 1875mhz for 850mv so that's what I've been running it at.
Here's a photo of my build: