That is, there is more CO₂ created at the refinery when processing higher-octane gasoline.

In any given engine, there is no appreciable difference in emissions from different octane ratings, but modern high-compression engines designed to run on higher-octane gasoline are, by there design, lower in emissions. It's not that the high-octane fuel burns cleaner, it's that the engine is designed to be more efficient in general. The high octane requirement is the result of the high compression ratio (to avoid detonation).

There is no benefit to running a higher-octane fuel than the engine's manufacturer specifies, either in emissions or power.