Low CR engines use lower octane fuel because the octane rating is about Knock AKA predetonation. Modern high compression engines have variable timing and ignition and so forth just in case you use lower than optimal octane rating so you don't do damage to your engine. This protection, however, will cost you some FE.
In summary, an engine that doesn't need higher octane won't become more efficient with higher octane.
HowStuffWorks "What does octane mean?"
Also some gas brands put additional cleaning additives in just their high octane fuel to help sell their more expensive dino-juice. Their are also top-tier gasoline brands that put additional cleaners in all their gas choices. But relax, in the US, their are additives that must be in the gas already.