Over the years, I've found VPower, and now VPower nitro to pay for itself in added fuel economy. My 911, and the previous 1 gained around 10% both urban and on a run over tesco 99. I found exactly the same with the wifes golf, which is also is designed to run on 98.
When i did an extended test drive in a macan (2.0lT), I took it to bournmouth for the weekend. (300m round trip)
On the way down, it was a 50/50 mix of petrol left in by the opc, and tesco 99. On the way back it was 75% vpower, and 25% residual mix.
The journey down it averaged 24mpg, and 27mpg on the way back. The traffic conditions were light in both directions, and the cruise was set at 80mph for 95% of each journey.
Engines designed to run on high octane fuels will run more timing advance, and more energy will be extracted from the fuel with less residual heat (energy) going down the exhaust. This is why they produce more power, and if the engine is tuned correctly, better fuel economy.
The fuel does not contain any more energy, but more of the energy is extracted by the engine, with less going down the exhaust as waste heat.
Ethanol content has a large effect on fuel economy, so some benefit could be from lower (no) ethanol content