once my S4 was mapped i have only used tesco 99, i cant really tell the difference in power as it was mapped so went alot quicker lol but yes it does help MPG, i was gettin 29 on a long run but now im getting around 31.9 on a long run with a map and 99.
for the extra couple of pence i would only put 99 in anyway. if you work it out over a full tank of fuel your spending a cou[ple of quid extra
on my S4 it has a 62 litre tank. normal fuel in tesco is 1.37.9 and 99 is 1.40.9 so im spending £2.48 extra on a full tank and im getting the good stuff