Also agree with Stubby. We drive under part throttle because that is the most efficient. Without tuning your entire exhaust, you will not gain FE simply by reducing back pressure after a primary CAT.
And I would love to add a second CAT to my car, if I could be confident it would improve emissions. Clean these gross polluters up!
Also, I started a thread on this topic a few years ago that was meant to try and capture the consensus of the best informed on this topic, on this site:
http://ecomodder.com/forum/showthrea...tml#post289125
Might be useful for you....