IMHO, the final analysis has to be made on a car that is running flawlessly (perfect spark delivery, perfect sensor system: knock H20 temp, O2, TPI, etc., perfect ECU maps, perfect fuel delivery, perfect electrical path tying it all together. As was mentioned, the Subaru ECU is pretty finicky about all of these parts working just right or it will start retarding/advancing timing to try to make it work right. Some of us see this in the low end of the RPM range, some of us see it in the mid and upper RPM registers.
The butt dyno is very subjective at best and, for that short burst while the ECU figures out what we've done to "affect" its internal maps, will probably indicate a "boost" in power related to the increased octane. Again, as was mentioned, that's only going to be temporary while the ECU relearns and returns what we've done to its normally programmed state. And it's pretty insistent on doing this.
Without beating a tired horse, I think one of the best ways to help the "system" to do its job is to make sure all the electrical pathways are free. I was tired of the mid-range performance dips and poor off-idle acceleration so I made sure that all the typical stuff (plugs, wires, throttle body, MAF,etc.) was working to factory spec and then started noodling around with different octane "blends". I'd reset the ECU, run two tankfulls and then compare the results. I never ran 92 octane (highest that's available here in SoCal) but the numbers just didn't change between 87/89 so I saw no need to pay the extra bucks for the 92 octane. My mileage and range remained consistent across the different gasoline mfrs and grades.
Once I did the single point ground (SPG), however, I had two immediate results. Smoother, consistent power delivery and increased mileage across all five Subes all running on Texaco (now Shell) 87 octane. I'm convinced that the sensors (particulary the knock sensor, O2, TPI and MAF) can now get their signal to the ECU more effectively and this results in more stable engine performance, smoother power and better mileage.
Hope that's not too off-topic but wanted to share my corollary to your octane thoughts.