Berk hfc ?
 

I know "the power of google"
I've searched hi and low for a definitive answer with mixed results.

On my 15 Z I have gen 3 cai with gemini exhaust.
Would I realistically gain anything if I put on berk hfc"s?

This car has not had a tune as of yet.

The factory cats are the most restrictive part of the OEM exhaust, so yes it would be gain. The car will get louder though. That would also be the threshold where a tune would be recommended.