Bro check the spark Plugs ( Clean or replace them if required), if they are fine, then add a bottle of fuel injector cleaner to petrol tank, before doing any tuning. Because i have seen many PW members complaining after such sort of cleaning.
Also when ever i visit Isb or pindi, i use hi octane only, just for the purpose of carbon deposit cleaning.
I also own vitz 2007 uplift model, i am using it from past two years, and trust me i am not getting any problem, mash allah pick is great both with AC and without AC, Mileage(11 to 13) is normal as i am using 15 Inch rim size.