The polo g40 digifant ecu will always stays at clossed loop (from the o2 sensor) and always hits 14.7 at part throttle conditions.
If you have a chip to suit 250cc injectors you'll be fine, with the new setup.
So the engine will be fine, only at part throttle conditions (off boost)
It wont be the same at wot conditions. The o2 is dessabled so, the ecu reads directly the values from the fuel maps, so the extra displacement and air feed from the bigger charger will lean the mixture, witch is not quite good...
Alex