I will just say what I've got from my own research here.
The card doesn't power throttle in any games I've tested, it always runs highest freqs without issues on stock core clocks.
I guess power throttling could appear only with overclock and on this point I will close out this issue.
This applies to only my own RX560 and I understand that it could be different on something like rx580.
Thanks for participating in the thread.