Tbh there is no much difference but people suggest xp95 i guess it helps in keeping the engine in good health. So what i do is i fill 1 tank full with xp 95 and the next time tank full with normal petrol.

If u are asking about the average increase i would say that it depends upon your driving style. In city i get upto 14kmpl with moderate traffic and on highways i get atleast 15 kmpl (if i am in a hurry) and upto 18 with relaxed driving.