I'd like to reopen this question. I'm on Big Sur 11.1 with a new 5600X on an X570 mobo. With AMDRyzenCPUPowerManagement.kext I seem to get no further than 25W minimum CPU power consumption, with temperatures hovering around 47°C.
However, in Windows 10 with AMD chipset drivers and the "Ryzen High Performance" power plan, AMD Ryzen Master shows me a consumption of only 3-8W when idling, with temperatures around 40°C. Also, the cores are reported to go into a "sleep mode" even though Windows task manager reports a clock of over 4 GHz.
I've found that in macOS the CPU never really idles and goes to sleep. The cores will hover at the highest speed step and only periodically go down to SS2 (never SS1). When I check the option in Power Gadget that's supposed to automatically clock the cores down, that rarely happens; basically only right after booting into macOS. As soon as I launch any application (and close it) the cores never calm down.
I'm not too bothered by it but I wonder what the point of "native" (UEFI-controlled) power management is when power is not really saved at all. And it seems that at least the CPU temperature has not decreased at all since I installed the kext; it was around 50°C before I had the kext running.