If you are saying that the difference between using RON95 and RON98 is within the error of a dyno, then why are you so adamant that there is a loss of power if you yourself say that it is not measurable by the dyno (which is the only way to quantitatively measure torque and power of cars). You are contradicting yourself.
And you say that in order to reap the benefits of higher octane, you need to be able to advance ignition timing. Yes, that is true and yes, there is a limit up to which the ignition can be advanced. The Euro has VTC as well which advances cam timing. It has been done by Hondata on the K series engines that more cam timing allows more power to be extracted by the engine.
So while I am not saying that using RON98 on the Euro gives more power, I am saying that it is possible to remap the ECU so that ignition and cam timings are advanced so much that more power can be produced. In order to run the engine at that ignition and cam timing, RON98 is needed.
RON98 does not give the engine power. It is the ability for the engine to advance ignition (which is what the monaro and K24 does) and cam (K24 though VTC) timing that gives power. As a consequence of the timings, you need RON98.
And BTW, A/F ratios, ignition and cam timings have been remapped for the K24 by Hondata. They did not produce a large increase in peak power (about 3hp at roughly 7200rpm) but instead produced a fat chunk of power between 5000-7000rpm. At 6000rpm, the gain was 28.8hp. So it is the total opposite of your claims that through advancing the timing that you gain only at one optimum RPM point.
All the dynos you could possibly want from Hondata on the K24:
http://hondata.com/reflash_tsx.html