My car:
Nissan Silvia S13 Sr20det 8psi
I picked up a used ISR cat-back exhaust for my car that i’m throwing on today, but it also came with a ISR downpipe. The Downpipe already has a bung on it which is awesome because I can finally run my AEM AFR. My question is...
If I’m still running the Cat can I throw on the down pipe? or will I need to get a tune after I put on the down pipe because of the difference in back pressure?

The downpipe and cat should bolt up. Some people run a test pipe with the down pipe in place of the CAT.

The back pressure will be less and you should gut the CAT or run a test pipe. You will notice your turbo will spool a WHOLE LOT QUICKER LOL. Yeah it makes a BIG difference. We tried it back in the day. My eyes got big when the hand went into the rev limiter quick af. Let us know how you are doing with it.

Yeah if you live somewhere you can get away without a cat, that's the best option.
A fresh tune after that certainly wont hurt. As mentioned, your turbo will be spooling a good bit quicker.