I don't believe that there is any truth in the suggestion they easyJet would avoid basing aircraft at SOU for fear of diluting their LGW pax etc. They'd quite happily expand their south of England business, and moving SOU pax away from LGW would allow them to attract yet more pax via their LGW base.
Clearly easyJet will move away from A319 in the coming years, and focus in on 320/321. As suggested these 320/321's may have performance restrictions at SOU, rendering it less attractive than other bases across UK and EU. Let's not forget the airport is competing with airports across the EU for based aircraft, e.g. NTE is getting a base, you can be sure that the region in France is sweeting a deal that SOU may never consider..
Another big item here is Flybe's controlling shrare at the airport; they are a big customer and can call a lot of the shots, you can be sure that they've brought their share to bear in terms of leverage...