Assalam o Alaikum,
Yes, Catalytic Converter is located in the Engine by the fan at PASSO.
That is one of the most important and expensive part though.
If you get this serviced and keep it maintained and your silencer + dholky is very well maintained as well then your car will give you very good Fuel Mileage. (Avoid to change the sound of the silencer, as it consume more petrol).
My Passo is giving me about
11.8 - 12.5 without AC at City (Its more better at highway)
11.2 - 11.6 with AC at City (Its more better at highway)
Remember:
I would advice, do not go to any unprofessional silencer engineer, as Catalytic Convertor is expensive and sensitive for Fuel Average.