Strange. My RX560 memory downclocks to 300Mhz but the core will only go down to 1.33Ghz. This doesn't affect thermals and the fans don't spin as you've described though. I'd assume you're using Whatevergreen? Have you tried using AGPMInjector.kext? You'll need to use the SSDT-PLUG.aml file so it'll attach to the X86PlatformPlugin though.
I didn't try AGPIMnjector as I'm switching to Intel in a few days (mainly for 100% Adobe compatibility, makes me kinda sad). I'll try then, thanks!
And because macOS gaming library without CS:Go/CrossOver is not really that big.