i did a lot of test with 95 and higher octane in my GD 1.4 when I had it (have Civic 1.8 now, but wife still has Jazz), and noticed the improvement in both responsiveness and MPG pretty quickly - without resetting the ECU. i got up to 10% better MPG and car definitely felt better. On the other hand, my wife with identical car used to use supermarket and her car never ran well until I persuaded her to drive mine and she saw how much better it felt even on normal 95.
remember 95 is 'minimum' octane for Jazz, it will run pretty good, but the knock sensors retard the ignition and you don't get as much 'bang-for-your-buck' as you do with higher octane, Some websites (especially in USA) will tell you there is no advantage to using higher octane in a normal engine because energy density of high octane is actually lower than 95RON because of anti-knock additives there is less petrol and more additives, but remember that normal compression ratio in USA gas guzzlers was about 6.5:1, which means they would run on kerosene (paraffin) anyway. The Jazz is pretty much 11:1 CR and definitely benefits from higher octane (and has the ECU and sensors to adapt to it). Trouble is you tend to drive in a more spirited way with higher octane which cancels out the MPG savings but definitely feels better.
