I dont doubt our potential but that's a bout 7% of the population of Sussex.
I don't doubt we could sell out 40k on the crest of a wave but generally where we are now is about right IMO
I think the fact you can still buy tickets for tonight's match (albeit not many) shows that the Amex is about right in size.
Also adding another 5k seats probably kill the exchange set up we currently have which seems to work pretty well at releasing extra tickets for occasional fans.