So finally went to Toyota S3.
Told them about average and everything, the guy (some manager or someone) said it's kinda normal, to achieve 8-9km/l if the speed is 40km/h in the city.
Got engine oil changed, they used Petron 20w-50 (asked the senior mechanic why not 5-30 and so on) and he suggested that I use the Petron as it can withstand max. temperature. Fuel filter and air filter were also replaced.
Senior mechanic took my car for a test drive (along with me) and when he came back, he said that engine was (a bit weak suggesting maybe CAT converter is choked) (also heard over his conversation with manager and he was saying to him like engine was 80% - but then he also said, all JDM Vitz have somewhat weak engines when they land in here in PK.
Anyway, everything else seemed fine, senior mechanic cleaned the CAT converter, used some silicone gel as he said there was some current leakage so hopefully RPM shouldn't drop.
Been two days and mileage seems improved to me. RPM also looks stable when I drive the car into my garage with AC on.
But still I feel (esp. when reversing the car on a slope) that it's underpowered (as compared to before my visit and all the changes they did).
So what do you think guys.
Thanks nawan885 for the update on your mileage, it's amazing though I usually keep the speed like under 40km/h.
P.S: Funny thing is somewhere deep inside I am thinking, well, let's save some money (and sell this car - though I love its comfy drive and just like it) and buy a sedan next time, maybe Prius or GLi AT.