RPM figures are nice but for me is really a small part of how I want my engine to perform. I would suggest the you run your tests without any adjustments to the engine other then mixture. stick with the same prop throughout. Log information such as ease of starting, transition, temps, vibration and of course top end RPM. Give hard figures such as temps after landing and RPM. Transition and vibration will obviously be more subjective. Once that is done do the same tests but try advancing the timing a tad with the higher octane fuels. if you really want to fiddle you could try some thinner base gaskets to bump up the compression with the high octane fuels.

Now for the disclaimer, for this to really mean anything IMO it has to all be done with the engine in the airplane and airplane in the air. Make the change, fly the engine for a gallon worth of fuel taking notes on handling charicteristics then at the end of the gallon record RPM figures. It will be a lot of work but at the end it will be good useful information. One thing I did notice of interest. Today when I picked up some Sthil HP ultra oil I read in the directions their recommendations to use only 89 octane. I wonder what they know that I don't
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