Forget any use of the 737-800NG or Max at SOU on a regular basis even with the tiny runway extension...
The a/c is s slippery beast, with much higher landing speeds, the brakes are not so fancy, plus other performance related issues for take offs that will not see this a/c be used at SOU for any high density loadings - If you then restrict payload the £££'s lost matter, which no airline will seriously consider.
If you think otherwise then why do you think that none have ever operated out of SOU as yet - and stayed...
The A320 is a different animal - still economical to operate out of SEN even with a slightly restricted payload of between 5-15 seats less (out of 186) to the Canary islands.
If anyone new is going to come to SOU to have a go then it will be EZY imho.
(I suspect they are watching closely as to what happens,,,)