Hello! You can apply the overclock to whatever kernel you want. It’s more, you may want to have an updated governor in an updated kernel to get the most out of it, re throttling etc.
The stock 0.5 image has the stock kernel, so could potentially lead to battery life being less.
Although are you specifically wanting to have 1200mHz and not 1400mHz?
You will need to replace the .dtb file in the boot partition.
It’s more, you may want to have an updated governor in an updated kernel to get the most out of it, re throttling etc.