The big difference between SEN and SOU (identical runway lengths after extension) would be the ground obstacles. Hills, trees and rail depots.... it's even listed as a hazard in the airfield guidance from NATS.
If the aircraft needs to ascend more quickly to avoid obstacles, then the aircraft cannot be heavy. Meaning any flights over 2hrs would be economically questionable.
Please don't get me wrong RivetJoint - I support the expansion as I would benefit from it too despite being a BOH 'troll'. It's just not as simple as SEN's extension.