rog747
The A320neo does offer better airfield performance although I don't know the exact improvement. The PMI-based A320 currently operating SEN-PMI frequently carries a full pax payload in summer conditions so the A320neo would open up more distant destinations.
As far as SOU is concerned its declared distances are very similar to those at SEN although the 20 climb path does have obstacle problems. A "15 degree slew" departure procedure is available on 20 but I don't know if easyJet SOPs allow its use.
Incidentally it was the fact that the 1-11 had to operate via OST to many Channel Airways destinations that forced the move to STN. Quite why they bought that aircraft knowing its severe SEN limitations, rather than the B737 and DC9 that they also considered. remains a mystery.