jzelectronic has the X300 L1.46 bios listed in their blog "31.12.20 19:13 BIOSe für ASRock Serie AM4". L1.46 has PBO, XFR, CPB, TSME, Global C-State. AGESA ComboAM4v2PI 1.0.0.2 SMU 4.30.71.0
I know this is the A300 thread, can't see a X300 thread like this one.
I got the X300, 3400G...