this has been covered to death
using higher octane fuel is fine
without a retune you wont get any gains other than it acting as a detonation killer
you wont gain any power without a retune
ie: putting 100ron in a stock GTST will give zero, unless you tune for it
but it will take away knocking if you do get some with normal 98 on your current setup
ethanol in a std car is fine it wont do any harm or melt your fuel lines etc
ive been using shell vpower racing 100ron for about 5 months (ish) now and its fine
i havent changed a single thing, std GTR pump, std lines, std injetors etc