janab aap apni gari k baarey main "touchy" nahi hain... 

yaar, jokes apart, ur idsi is tuned to idle at 700rpm on cng... thats why the idle rpm on petrol will be a bit higher than that... Fahad bhai's car is on petrol...
Mine idles at 680 on cng and around 720 on petrol... doesn't make much of a difference to me... 
haha... :D:D
Dude, u've put the best answer to ur query in ur own post...
People like you who drive less than 800kms in a month, IMO should not worry about fuel average. More or less, it does not effect ur budgets to a cosiderable extent.
People like me whose cars are driven like 4000kms intercity, and another couple grands on highway, have these worries... 
Just enjoy the extra grip... don't change the profile. Won't make much of a difference at all.
My vtec city is driven like 40 kms a month.
So, even if a drive it sometimes,I push the limits... (H)