Octane all has to do with engine knock/preignition on the compression stroke, the higher the octane the more compression it can withstand. The 91 number is relative to which country you are in as the testing/rating is different here in Canada & the US vice Europe. So could the performance difference be noticed if one does not experience "engine knock"?? I wonder if the owners manuals state the octane number differently in Europe vice the US??
I have 74mm throttle body and NW CAI, use 100% gasoline (no ethanol) and 93 octane; runs good.
I doubt it would be running lean on 89. The ratio between fuel and air should remain constant with the tune. If the fuel does not ignite early because of compression temperature then I doubt performance would be effected. I guess one might be able to find the difference with a dyno comparison, but if there is no preignition and you are happy with the performance...Carry on. It is your choice. If you race the car then higher temperatures would be expected and higher octane would be required.