As I keep saying to the Luton folk capacity is set by the weakest link in the chain, whether that be the runway capacity, taxiway capacity, the apron space or the various parts of the terminal like the number of check-in desks and baggage belts.
There seems to be quite a lot of spin in the article. Comparing Southend to Luton is like comparing Luton to Gatwick.
I doubt very much that the capacity of Southend's runway will be anything like 36 movements per hour due to backtracking. Luton is only achieving that sort of figure as most aircraft no longer backtrack but I stand corrected if the opinion here is that 36 movements per hour can be achieved when either runway end is in use.
Also the passenger experience will be somewhat diminished if Southend was handling 5 million passengers rather than around a million passengers it handles now.