There's a misconception regarding fuel grades and how they are worked out, most people are led to believe RON ratings are purely power gains and using a higher ron rating will boost power or gives you extra power, but basically the higher the ron rating the better combustion it gives in the engine, but not all cars will run better on higher ron grade, my 2.2 vectra ran much quicker on shell vpower, but wouldnt on others like bp ultimate and so on, put some STP petrol additive in too and it was quick as hell, you can run a car on a higher grade fuel but will give different results from which brand of petrol you use, my vectra diesel runs quicker and smoother on bp ultimate and my partners 1.8 vectra petrol prefers esso energy.
95RON is the minimum sold in the uk purely for regulations (why exactly it varies from country to country i have no real idea)
You are right in saying its been detuned to accept 91ron, if we used 91ron in z22se/yh the combustion would be too week and probably experience some knock or pinking, because our ecu map is allowing for a minimum grade, exactly the same in australia, if yours is tuned to run on 91, and for example you had 87ron to put in you would experience the same.
As far as what you need to change aswell as the ecu im not too sure, i would have thought ecu, transponder and chip, maybe @[deleted] or @[deleted] could help with that one. also they may be able to give you a second opinion on what i've said.