Update on my Passo testing:
Last week's average mileage: 12 km/l, normal acceleration, moderate traffic city driving with AC on, O/D off below 60 km/h.
Not very good performance. And I had already changed the engine oil / filter and the air filter last week.
Today, I took the car to Toyota 3S, they did a full engine tune up. Found all spark plugs worn out and leaky, got them changed with new genuine irridium ones (cost Rs. 1200/plug), got throttle body cleaned (was getting blocked with carbon), and got wheel balancing done. Total bill = Rs. 6,500.
Asked about transmission oil change, but the Toyota guy told me there is no need unless you feel the gears are not changing on time or properly. Generally they change it after 80,000 km for automatic trans. Mine has done only 30,000 so they recommended me against it.
After tune-up and everything, now I can feel the car is more responsive and is giving a more powerful pick. Will run it through the same mileage test this week, however with O/D continuously on and will report progress enshallah next week.
Tip to folks who have done everything and still not noticing improvement in mileage: Try getting your wheels balanced and drive with firmer tyres (~ 35 psi inflation pressure instead of 30 psi). Let me know if it helps.