Yeh where else is it going to read from? I cant see why the reading would be any less accurate than you wideband. They are both metering the same gas in the post turbo exhaust, just from different ends. Dunno if a cat would make a difference, best thing to do would be to get Mao to measure your afrs with his sensor and compare it too your gauge. And for the sake of a fair experiment and to rule out poor calibration of Mao's sensor or your gauge, take it too another shop and get them to check the AFR.
Quickly google'd Catalytic converters affect on AFR's and found this good article with actually testing. Read the summary at the bottom if you cant be bothered with the complete article
http://www.dtec.net.au/Lambda%20Test...0Converter.htm