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I'll post more later. But for now, general rule of thumb: Higher octane = less energy... It has less kinetic energy, not to be confused with the engine's being able to make power. Ideally, as I've said before, you want to run the lowest octane possible for your setup with the least amount of ping. Running 105 octane fuel on a stock 7M-GE will result in terrible mileage and even worse performance.

It's a balancing act and there's no simple cheat beyond keeping combustion chamber temps down (Ceramic coated air ducts, good IC, air filter not in the engine bay, EGR system intact, coasting with the clutch out, water/meth injection) and then run only the octane you need. I live a mile higher than sea level... I've run 87 octane on a few higher rated vehicles and found more response with no discernible difference in the data logs as far as the ECU adding or pulling timing.... Now going from a recommended 93 to 89 or 87 is something that's safe here due to the altitude and you can find publications by various manufacturers and oil companies stating what octane to use for a given altitude. Factory recommendations assume sea level. Many newer vehicles are better able to ride that performance threshold though and are able to make good power while maintaining equal mileage in which case I'd suggest sticking with premium if you can afford it.... but the MKIII isn't that smart. Normally I still run 91 octane (or 89 when I can find it. 91, 87 and 85 are the common ones here) as it's cheap insurance and I'm not trying to squeeze every bit of performance or mileage out of my car.

A code 52 will SLAUGHTER your mileage! Fix it ASAP.

Seriously though, your mileage doesn't sound off to me at all. I don't know how your driving style is but city is always the worst.

Do you have any means of fuel management installed? If not, add that and tuning to the list.