I think you talk about RAM clock speed and not CPU clock speed. The RAM speed can not be changed as it’s hardcoded by the bootloader. There is no real “GPU”.
You are able to overclock the CPU by config.ini, yes.
EDIT:
I am sorry, maybe I was wrong. There is a gpu clock in the dts:
But it include already 850 with a clock of 846000000Hz.
But no idea where and who does choose it.