Experimentation is definitely a large part of the hobby.

What you'll run into here is a weight problem. The wing is unreinforced foam. Double the battery capacity, and double the weight of the battery... On top of that, the battery is stuffed in the nose of the plane. More weight will make the plane more nose heavy, something it doesn't need to be.

I think you'll get an appreciable increase in flight time by using some quality cells instead of the cheap Chinese knock-offs that came with the Aerobird. Those 900mAh cells are pushed to the max, and lose a significant amount of capacity through heating.

Take a look at an "identical" pack made from KAN1050 cells. They've got 150mAh more capacity, but are much higher quality cells. They stand up better under load. You'll get better performance, and less loss through heating.