Yup. Choking / Clogging of Catalytic Converter is there. 02 main reasons are: Bad quality of fuel and stringent emission / exhaust standards in Japan and European countries when compared to Pakistan (like Euro 4, 5 and 6, when compared to Euro 2 here).
But nothing to worry, if it had been such a major issue, then all the used JDMs had been a big flop and standing in garages / scrap and junk yards, instead of roads. Its not that severe.
Proficar, Liqui Moly Cat Cleaner products, Motor Clean Pakistan, etc. are there to take care of these. Just get it cleaned / serviced, when you find it clogged, and then you are good to go.
Car sound will only change, when its "Jaali" is broken, severely damaged / deteriorated, or catalytic Converter has been removed altogether. It is always better to never let anything like this happen. Just get it cleaned timely / proactively, and its good enough.
AC of 1000 cc JDMs, like Vitz, Passo / Boon is Zabardast (my personal experience with Passo). No need to worry about it, however, for 660 cc cars, people say that AC does not work efficiently in summers.