Legend_Jr

I've been to a few shows and seen some mustangs of all kinds, ranging from fox bodies to the newer S197s, that have custom cat placements. Not attached the headers like usual, but further down, either right after the mid-pipe or a little further.

Is this legal with CARB states, and does it make any difference in sound or (if possible) performance?

09GT/CS

It's definitely not CARB legal. Moving cats from the "certified" location is against the environmental protection act. However in non CARB states they enforce that rule at their own discretion. Here in GA as long as you have something that looks like a cat and the PCM is happy then you get your printout and your on your way even though your technically in violation of the law. As for performance it really won't make any difference unless you are running long tubes and moved them back for that reason. Also they may plug up due to not getting hot enough being further back. Bottom line if your not adding long tubes there's really no reason/ not worth the risk to move them.

Siber Express

The Law basically says it has to be an OEM Cat in the OEM location. That being said a friend of mine got a ticket at a Drunk Check point in his truck for "Other than OEM cats", this was after his truck had been smogged and passed twice. $1500 Dollar fine and $1000 in parts plus the labor and then selling his truck because he had to pay for inspection anytime the feds wanted it.
To me, not worth it.