Generally, the higher the octane level, the lower the chance of detonation. Detonation damages your engine internals. Your car WILL run on 91 octane, but it's not smart to do so. If you HAVE to run the car on 91, then keep it below 5000rpm and lay off the boost.
The difference is only 10c per litre... Thats only like $6-7 per tank... Buy "home brand" bread and milk instead and give your car some decent juice man
The quality of fuel in Japan is higher than here too. Australian premium fuel is pretty much comparable to standard fuel in Japan from what i understand. That means that our standard fuel is definitely not sufficient to run a 'performance' car built in Japan, and designed for Japanese fuel.
Hope this helps..