It's physically possible to extend the runway at the eastern end but to be of any use it would mean dropping a main road under the runway extension and taking possession of a common.
The environmental disturbance and cost that this would engender was deemed by the airport not to be worth the likely result when it published its master plan several years ago.
Just to confirm, when I said it punches above its weight I was speaking of the physical limitations. The size and make-up of the catchment would support a busier airport (it's at 6.3 mppa at the moment) if it was more conveniently situated and on a larger site.
Realistically it has to make the most of what it's got with a current limit of 10 mppa placed on its planning consents.