The book Soccernomics has an excellent part on stadium expansion and proves the old adage "if you build it, they will come".
If we expanded St Marys and were able to buy some of the surrounding land for development (think the area around Wembley, MK Dons stadium etc) we would absolutely draw more people in.
Personally, I think it's worth adding another tier on top of the Kingsland/Northam just to shove the away fans up there and finally have the Northam just for Saints fans...