I do get clock stretching behavior at 6400 Mb/s. I have not tried to isolate memory, though, so it could be CPU/MC causing it. Also @ Infinity Fabric 2200 MHz, the CPU appears stable but Bluetooth audio becomes wacky. These are not unexpected behavior since the hardware is pushed to limits.
Another discovery: Samsung dies actually use less power than Hynix dies in idle. It's something like this on my system when memory VDD and VDDQ are set to 1.25V
Samsung dies: Idle 0.125 W Load 3 W
Hynix Dies: Idle 0.5 W Load 2.4 W
Load is during Prime95 LargeFFT AVX-512 runs. Another fun fact: These sticks can be arranged in any order in the board's DIMM slots. It could be 1-3 / 2-4, 1-2 / 3-4, or 1-4 / 2-3.
I think I am pretty much done fiddling with the system. Despite many complaints I shared publicly (from pricing to the operating temperature), I am rather pleased with the whole AM5 experience. The products' fit and finish is excellent, especially for a brand-new platform. I rarely ran into an issue that is not of my own making, and even then the platform handled most of my idiocy with grace. Silicon quality also appears good, no doubt thanks in part to TSMC's now-mature 5nm process, which was not the case with my first 3700X years ago. That AMD had to push the clock/power to such a degree out of factory is unfortunate, but still, compared to the 13700K I had until very recently, this 7700X is cooler-running and downright frugal in power use.
4 DIMM DDR5 stability and performance is top-notch, which is an unexpected (and a pleasant) surprise. It did not require any special setting in the BIOS, I just had to take into consideration that I am using memory sticks made from 2 different kind of ICs with different characteristics, meaning various parameters should be catered to the lower denominator. Performance loss incurred from 4 sticks (vs 2) outside of some memory-centric benchmarks is non-existent. But you have to keep an eye on the temperature under load.
Ideal configuration is running 3:3:2 ratio for Memory Controller : Memory : Infinity Fabric. Actually Infinity Fabric is "decoupled" per AMD, and from my testing that appears to be largely true. Again, outside of memory-centric benchmarks, Infinity Fabric 1800~2200 MHz impact on performance was minimal. I believe this is mostly thanks to the enlarged L2 cache masking memory latency better. Overall, my observation is that memory clock/frequency are less impactful on AM5 than on AM4, or on Socket 1700 for that matter. You simply want to match the memory controller speed to your memory sticks, and let the motherboards take care of the rest.
Speaking of motherboards, despite my gripe against the pricey nature of all things AM5, I have to say the experience has been honestly excellent. With AM5 AMD set out to be a high-end option instead of a budget alternative, and in my opinion they met that goal, from product quality standpoint. My understanding is that AMD imposes certain minimum requirements for current AM5 chipsets to motherboard vendors, such as PCB layers and VRM thresholds, and that is a big part of high price tag of current AM5 boards. Perhaps future AM5 CPUs are also in the mix of calculus, since these boards are supposed to be supported throughout years to come.
I still think there are some undeservedly expensive motherboards out there, but after having experienced the platform first-hand, my initial anger at the price tag has been somewhat neutered. If one shops intelligently, I do not think anyone will be disappointed. Hopefully as production matures the prices will come down in the future.
Overall, I am happy to have my first 5.0 GHz CPU from AMD, accompanied by fittingly nice platform. My gosh, it's been only 11 years since I ran my 2500K at 5.0 GHz.