[Zeke]

AFAIC, by the time the exhaust gases get to the cat, things are out of balance and nothing will make much difference other than less back pressure. An old clogged cat can be problematic in that regard. If your cat is restrictive, either choice will make a little difference.

I'm waiting to see a better aftermarket system for the 3.0-3.2 engines than what I have seen so far that is CA legal. I think dual cats is part of the answer.

You don't say where you are, so I don't know if that makes any difference to you.