Putting aside what you say about dyno comparisons (which is fair enough)- you are the first person I have heard say 98 ron will produce less power than 95 ron in a car rated as 95 minI
I have definately heard people say (like Wyn) it may be no better unless you get an ECU remap, etc - but saying it will produce LESS power on the stock ECU tune - is news to me.
Eg - look at this article - my understanding is the MY99 WRX was tuned for 95 ron just like the Euro. Accepting what you say about dyno comparisons - it didn't show less power.
http://autospeed.drive.com.au/cms/article.html?&A=0429