So overall the gaming PC has been running great, and temps are within spec in all games EXCEPT Control and Metro Exodus, both of which are running DX12 with RTX ray tracing and I just started playing again. The CPU is the problem. The GPU temps never come close to max (love those EVGA coolers).
Control in particular uses a LOT more CPU than most games (I've seen it run as high as 40%) and with the RTX 3080 maxed out and dumping a lot of heat into the case, the CPU temp on the 5800x easily gets up to thermal throttle at 91C in short order. The throttling isn't bad, maybe 700 MHz, and it doesn't dramatically affect gameplay, but I'm leaving performance on the table, and running things needlessly hot.
If I had to do this build all over again, I would probably use the pricier and beefier Noctua NH-D15 CPU cooler that Err recommended, but I'd hate to pull the mobo and change coolers at this point. It's not that the Cooler Master Hyper 212 isn't adequate. I can run Prime95 all day and the CPU won't go over 85C, but with the added heat dump from the RTX 3080, that drives the temps even higher. It's possible even that monster NH-D15 cooler would have the same issue: just too much heat rising from the GPU.
So, step one: I needed a reliable way to test temperatures so I could compare any changes I made. I chose 15 minutes running Prime95 at the same time as the Unigine Heaven benchmark. This puts both the CPU and the GPU at 100% load, and very quickly reaches a stable temperature. Now this is a totally unrealistic test - no game will ever run the GPU & CPU both at 100%, but it is the worst case scenario, and easy to reproduce.
Step two: I added a pull fan on the back of the CPU cooler, which I didn't expect to do much from what I've read. It did help a bit as it did take longer for the CPU to hit max. Maybe realistically it knocks off a degree, but I'll take that.
As an experiment I left the front panel off, and sure enough, the system ran extremely cool. The CPU never hit max under full load. Not very practical maybe, and certainly not pretty, but I wouldn't totally rule it out as the PC sits under my desk out of sight. However this did convince me that for my next build, I will look at cases with whole front intake vents instead of intake vents just on the side of the front panel. I've noticed a trend towards those type of cases lately with these monster space heaters we call gaming PCs, and away from blank or glass fronted cases.
But this got me thinking about airflow, and I've been watching WAY too many Jayz2Cents videos lately about air flow. The case has an unused DVD drive bay at the top. Normally a DVD drive would plug this hole, But on my case I just have a 3 1/2" HDD in there for image backups, and it's mostly open to the area in front of the three front intake fans, with a black plate plugging the face panel. That seemed to me to be a path that would allow hot air, at the top of the case, to be pulled back in via the front fans. So I taped over the DVD drive bay with some non-gooey blue painter's tape, and noticed a substantial improvement in temps. Still the CPU would eventually hit max with my test setup, but it took the full 15 minutes.
I played Control for half an hour and it never exceeded 88C on the CPU, so it seemed to be the fix, but I wanted to see if I could improve airflow even more and keep the temps lower. What if I removed the blank plate in front of the DVD drive bay to allow it to suck more air in that way?
This made a big improvement. I can feel a large amout of suction in that open hole, and temps are now a lot lower. Even my brutal 100% CPU & GPU load test won't drive the CPU up to more than 88C.
I ordered this filtered cover plate for the drive bay to make it look better, instead of just having an open hole, and to keep dust out:
https://www.amazon.com/dp/B017TM7S9W