My experience may be a little different than most of you, but it should be relevant nonetheless. I dont have a large scale RC, but I used to own a mostly stock Martin Monster 23cc scooter and I have run high octane fuel in it with amazing results. Results based on a cheap Bushnell radar gun, tuning the mixture on the fly for each run, with several passes for a fair average.
Pump 87 octane+Tech 2000 2-cycle oil+Me= 23mph top speed
VP 118 octane+Royal Purple 2-cycle oil+Me= 27mph top speed
Out of all passes the smallest difference was 3mph, average of 4mph difference from several passes. This might not seem like much, but dont forget it was carrying me, at 190lbs. I imagine in a RC, this could translate into much larger gains. As a certified auto tech, I have always understood the same theory that Ridley has posted above. But experience tells me the engine manufacturer may not always have it right, and it might be worth a shot. I have seen some full size cars experience large gains in fuel efficiency and power from higher octane fuel when the manufacturer recommended 87. Yet other cars have shown zero benefit from the same treatment. As I said, not necessarily a direct comparison here, but it should be relevant to the discussion.