I don't think the cat would affect your AFR readings that much. I've used a tailpipe WB on a dyno and then a wideband precat on my car and the biggest difference was the delay at lower rpms since the exhaust gasses had further to travel.
If you download a free logger, like from tari (tari.co.za) and go to the ecuexplorer forum. You will find the latest release there. Install that on a laptop and you can use the cable from your accessport to datalog various datapoints on your car.
I would look at the long term (learning) #1 fuel trim at idle and while doing some light driving around town. If it goes much further out than +-5%, it could be indicative of a problem. If you want to datalog, your long term, short term (correcting) fuel trims, maf voltage and rpm and send a log, I'd be happy to help you look at it.