Well, checked my manual last night and it advises at least 95 Ron. Do you think that the Ecu might have a large enough band of settings to avoid the need for a remap when using 98?
definately not, the ecu's map is designed to allow a variety of fuels to be used, but it can only adjust ignition timing from the default when the knock sensor tells it the fuel is preigniting (pinking, knocking, whatever you want to call it). that means a lower than expected octane will result in an adjustment to the ignition timing (so you can use crap fuels abroad ok), but once the fuel is no longer preigniting the ecu will resume use of the default map's ignition timing, which is set up for 95 ron. a higher octane will not result in preignition so the default map will still be used, there is no way for the ecu to know that it can retard ignition further so you get no benefit from a higher octane.
think of it like this. the ecu will constantly adjust the ignition timing up to the default map settings, but not above, that is why your default map must be set up to take advantage of the fuel you will be using.
most JDM vehicles are set up to use 99ron, their european counterparts are usualy set up for 95ron. same engine, different map. if you buy an import JDM you are stongly advised to use 99ron, if you have the uk model there is no point.
so the natural question now is, why arent all cars set up for 99ron so they can all take advantage of retarding the ignition to suit? and why must you use 99ron if the ecu's map is designed for it, whats wrong with 95 ron?
there is a very good reason. when you run an engine on an octane that is below the octane used for the default map there will be almost constant preignition as the ecu constantly adjusts the ignition timing in an effort to get back to the default map timing, that causes almost constant preignition but it does not stop your engine from running (so you can get home on really crap fuel if needs be) but in the long term it will result in premature engine wear and a loss of fuel economy because preignition is not a good thing.
car manufacturers know that in the european market the most likely fuel to be used is 95ron, so they set the cars up to use that fuel so that in normal operation your knock sensor will not see any knock at all, which is good for the enigne in the long term. a JDM car is designed for 99ron because that is the most commonly used fuel in japan. if you run a JDM on our 95ron it will adjust the ignition timing but it will also be constantly trying to get back to the more retarded timing for 99ron, and that constant adjustment (and the preignition it causes) leads to engine wear in the long run, eventually having an effect on compression and hence performance.