The engine management system in these vehicles constantly adjusts the ignition timing to suit engine load, throttle position, ambient air temp, octane rating of the fuel used etc etc. . The engine can take full advantage of any higher octane fuels if used.. The ECU continually monitors the engine for pinging/knocking/detonation via the knock sensors, and if it detects any, it will pull ignition timing out until the pinging/knocking stops.. Usually the higher the octane rating of the fuel, the more ignition timing can be used before detonation/pinging/knocking occurs under power/load. The more timing advance you can run, (to a point), the more efficiently the engine will run, which means better economy, more power, and cleaner emissions and better fuel economy.
To see just how much difference the engine management makes to these engines, You only have to look at the engine bay of a VL commodore and compare it to a VN, the VL has hundreds of pollution pipes etc running all over the place, whereas the VN has bugger all pollution systems, this is all because of the engine management system, which controls the engine parameters way more finely, which in turn, greatly drops the emission levels and increases the engines efficiency etc, so theres no need for all the pollution gear like on the older models.
A tune on these vehicles will basically give the ECU access to higher ignition timing values..
With my VS V6 I used to get excellent fuel economy just with the standard tune and running 98 octane fuel, but over the last couple of years Ive done a hell 0f a lot of experimenting with different timing values in the tune and I have got the fuel economy down even more, around 6 litres/100km highway, and around 8-11 litres/100 km around town. Thats on 98 fuel, and driving fairly normally. The tune hasnt made the engine more powerful as such, but its definitely a lot more responsive than before, especially noticable when accelerating from light to mid throttle.