i dont think the cat changes how the car runs so you might want it if you need to do an emissions test. Thieves cut them out quite often. if someone ripped mine I'd replace it with some straight pipe. New cars have a o2 sensor to cause trouble lights to come on so you run to the dealer to spend money. as far as I'm aware the only sensor is the O2 sensor before the cat. They did away with emissions testing here.

i seem to remember a gasket that is shaped like an O and made of something similar looking to asbestos. if it should be there the parts diagram will show it. I think we should check if there should be a part between. If that is missing maybe that's why the flange doesn't line up right?