I simply cannot see SOU ever achieving 6 mppa.
Just where would that level of traffic come from? Its natural catchment simply isn't that large and Hampshire only has limited appeal to the inbound market. They would have to offer some fairly spectacular destinations in order to attract pax from further afield and their runway is likely to preclude that. The only time I flew from SOU was when going on holiday to Alderney and I drove down from Norwich, so that is one such route but I cannot think of many others.
At least BOH has greater potential for growth as it has few constraints on aircraft size and route distances, but again it's growth is going to be constrained by the size of its catchment and inbound traffic is unlikely to be significant.
I appreciate that SEN is slighly off topic here, but as teleparty and RooCat have offered their opinions I would say that South-East Essex has at least as many large employers of does the Bournemouth area, with RBS on the airfield boundary and KeyMed a mile away being just too examples plus the Basildon industrial/commercial centres and World Port being built a few miles down the road.
The real point though is that SEN has London on its doorstep and inbound traffic will play a large role in its growth while the direct rail link will also be drawing in outbound traffic from East London. Add to that the 600,000 local population for whom SEN is 20 minutes travel time closer than any other London airport and its potential becomes more apparent.
But time now perhaps to leave SEN out of the debate and get back to BOH matters.