... once a gallon of gas of any grade 87-94 is consumed, the total VOLUME of exhaust gasses is pretty much the same, or perhaps greater with higher octane, especially if the octane is derived from MTBE and/or BTEX instead of ethanol. What matters is what comes out of the exhaust pipe and MPG.

Here's what's supposed to happen with ICs in theory:

2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O

But, there's other stuff in gasoline besides C₈H₁₈. Maybe a touch of S.

And there's other stuff in the air beside O₂, mainly N, plus the ambient pollutants borne in the air, which will vary by locality.

No matter how you treat exhaust gasses, with catalytic converters and or octane rating, what goes into combustion is the chemical formula of gasoline and it's family of compounds, plus additives and any impurities not removed during refining, plus all the stuff that's in the atmosphere, plus any tiny amount of oil that the engine might be consuming, and even when combustion is theoretically complete, what comes out is not necessarily pure CO₂ and H₂O.

To the extent that the gasoline is not "pure" and overall system is not working perfectly, you will also emit H₂S, SO₂, CO, various NO_x, Ozone, Formaldehyde (CH₂O), various other HCs and other pollutants.

(Not an authority on IC, but I took AP Chemistry, Quantitative Analysis, Qualitative Analysis, Organic, and AP Physics at Vandy, so I know enough to be dangerous )