Since it is a new feature you will be activating, it will consume the CPU a bit but that depends on traffic types and polling interval being set.
You may consider around 5 to 9% of CPU consumption max for Sflow typical traffic. However, you may configure the higher sampling rates just to see how it suits your environment to start with.
If you have any further questions, pl post your complete config and show sys proc extensive, show chassis routing-enginer output for analysis.