I've heard it changes A/F by .5 from a friend with a Dyno. I've heard from others that it can be done too. Honestly, I don't know how this could be as I thought the cat would eat up some gas if the car was running rich (and get HOT doing it) but the experience of others seems to suggest that's not true. To be 100% sure put it in front of the cat but expect added labor costs and grumbling from the operator. Some dynos, like my friend's, only have a tailpipe sniffer and cannot plug in ahead of the cat - you have to run it at the tailpipe.
I'd love to hear a definative clear answer on this along with an explanation that makes sense as to why it does or doesn't work.