the sensor doesnt go in the middle of the cat.
the first sensor goes before the cat, mines in the downpipe, then the gases flow through the cat which go through a chemical change to lower the harmfull emisions (or something like that)!! and then the gases flow out of the cat nice and clean and past the sensor which checks that they are different than they was at the first sensor. obviously no cat means the same gases are detected at both sensors so it throws up the engine management light. im pretty sure this affects the mixture because it probably adjusts fuel to try and change the gases as its confused. (this is my understanding, not the most technical but its somewhere along these lines)
using the angle sensor mod takes the sensor out the direct flow of the gases and not as much gas reaches the second sensor as its shrouded by this angle block, so it detects less gases and thinks the cat is doing its job. id say it would
help your fueling
problem if it IS caused by the decat and you never had this
problem before. personally id remove the decat and weld in a sports cat, easy to do and then you wont have this
problem, your car is probably running worse with the decat because the fueling is all messed up, mine run like crap