Cat converter has to be cleaned up. A good silencer guy can do that , he will remove the manifold and break the honeycomb inside and will remove the glass wool, but after this the car will develop a little more sound but not much, for that you can ask the silencer guy to add a dholki near the mid dholki. This might cost you Rs 1800 for cleanup and new exhaust seal and another may be 1000 for a good quality new dholkee.
But as far as I know that is the primary sound and heat supressor not a CAT. I may be wrong.
As far as mechanics are concerned. Computer diagnostic will decide what is wrong , and then you can have a second opinion.
Some times mere configuration helps.
for a few days . Fill up your tank and drive it very softly , you will find out that the millage is about 14 km/l with AC after that think about CAT