I think your wing loading calculation is wrong.
73 oz for 560 sq in gives 73/560 = 0.13 oz/sq in or 18 oz/sq ft.
Extra battery capacity does have a downside in that the plane is heavier so it need more power (and thus use more battery capacity) to fly. Add too much extra weight and the plane's handling can be impaired as well.
You are using the recommended motor and ESC so I would suggest using the recommended battery (2600mAh 4s) as well at least to start with.