A lot of cars now have a Sport button and in a NA car like e.g. the M3 it gobbles the juice reducing 'economy' by about 3mpg.
The Sport button on the A500 is similar to the 'more hyper' Dynamic mode on the MiTo and like any turbo charged car the boost is increased along with a mapping change i.e. this is in effect the 'Normal' mode on the TA.
From a theory point of view more boost means more fuel but it can also be more efficient than having less boost and having to use a heavier right foot to compensate. Some modders who have chipped their boosted cars noticed an improvement

in fuel 'economy'.
If you driving style is to 'nip along' then using the 'free' turbo power is a lot more efficient than having to crank up on a larger NA engine with equivalent outputs.
Have seem some reports of 'brisk' TA drivers getting a better mpg with Eco off than on but it may well come down to one's driving style.