Putting in a downpipe on my A4

Hey everyone, I was thinking of putting on a downpipe for my 2018 A4, but I live in California so I'm scared that if I don't have a catalytic converter I'll get pulled over. So if I'm putting in the downpipe. can I also put in a high-flow catalytic converter?

Here are the links to the parts I was thinking of buying:

https://www.ecstuning.com/b-ecs-parts/b9-20t-t304-stainless-steel-30-downpipe/021616ecs01kt/

https://www.ecstuning.com/b-cts-parts/cts-turbo-high-flow-cat/ctsexhtp0006-cat/