First of all, I've tried hi-octane, there was very little difference. I got about 5km more than usual from about 7liter fill up. If you can justify paying nearly 30rs more for such minor improvements then that's entirely up to you. Otherwise I'd recommend staying away from it.
The City (in its own class with 1.3l) is one of the fastest and most economical cars available. I usually get 12kmpl in city driving when driving neatly (but my car needs lots of tuning and other services!!) Any other City should get minimum 13+km/l inside the city. I measure this usually from fill-up to fill-up (not just off of the meter). Meter can be very unreliable, especially if you don't have the standard 175/65/15 size wheels fitted or wheels with a similar circumference. Changes in this total wheel circ. really throws off the trip meter so be cautious when buying new tyres also! Always buy 175/65 for 15" rims, or 205/45 max for 17" rims!
Majority of the time there is only one person other than myself as the driver in the car. When comparing with or without the load of the A/C on petrol, the average only drops a very small amount. And on the highway, not much at all in fact with A/C on! On highway expect to get 18-19km/l at speed of 120km/h.
I live in Fsd and our roads are not up to the Isb standard, but i think this is outweighed by the fact that Isb has busy roads anyway!
Btw NEW 1000cc Vitz gets 15km/l in the city no problem! (When i mean new i mean the new shape that is just starting to appear in pakistan very recently!) What model is yours?
Hope this all helps!