I've got a D15 and a 9900K running at 4.7Ghz on 1.20V. I've got idle temps at 35C. Case is BeQuiet Dark Base 900. It is closer to me than my old cheese-grater MacPro and runs as quietly. I'm sure that I could bump up the voltage and get it a few hundred Mhz higher, but 8 hyper-threaded cores at 4.7Ghz? It's not really a place to complain much, and it is definitely safe, long-term voltages. I ran a bunch of Cinebench runs and some other CPU-crusher type tests to make sure it was stable at full load. The fans get noisy and temps went up to high 70's to just over 80C as the load persisted, but stayed stable. Used as a DAW, it never hits 100% utilization. These CPU's run out of real-time capacity long before they run out of absolute capacity. I can hear the fans kick up when it is rendering a full mockup, but outside that, it just does its thing. If I took some things off of it and put them on sample servers, I'd get back some of my real-time headroom. It is a lot to ask a box to process a ton of dense MIDI data, soft synths, mixing, networking, etc. Eventually, they just say, "Enough!" no matter how many Ghz you have.
That said, Mishabou's report is the second I've heard of someone running giant Xeon systems with 512GB RAM. One of the composers on Spitfire's "Cribs" in Hollywood had also switched to that setup.
The trouble of course, is that none of us know how other people work, how their templates lay out, and how much load they really put on the box and in what way. How dense is the CC data? How many plugins, nested busses, etc. These things all really matter in the loads that sample servers and DAWs experience. The benchmarks give directional guidance, but I think there is a lot of variation in how people use this stuff, even when it sounds like we are doing similar things with similar vendors. For example, chillbot and I both have digital mixers. He mixes on them. I just do headphone mixes and auxiliary I/O on mine, and VEP audio comes back into the box. This puts a very different load on my DAW CPU than his. I think it is easy to talk in general, and expensive to get specific experience (it takes a while to get a template and VEP boxes working to optimal condition).