It may well be before the cat - is it in the downpipe?
The cat will affect emmissions if removed. The lambda sansor reports back to the ECU the mixture level, so if emmissions are high, the mixture can be tweaked by adjusting the lambda signal sent to the ECU. Depending on how high the emmissions are, it may be possible to bring them back down by fooling the ecu into thinking its running too rich. This is possible, I knew someone who did it on a Cavelier to get it to pass the MOT without a cat, regardless of the position of the Oxygen sensor.
Websites are like ********s, everyone has one, and im wary of any bloke trying to direct people into their's