Like thay say there is no free lunch.

More power less flight time, and viceversa,.

Power is in relation to the mass of the plane, so more wheight more power needed.

Increasing capacity icreases the wheight so think this is not the way.

So possible solutions

1. A more eficient motor,(expensive and think there are only two option scorpion, and litlescreamer)
2. Wait for the new contrrolles that will handle more cells, so more voltage less amp draw and needs less capacity batteries.
3. Reduce wheight of plane, 10 cells instead of 12,
4. Proper throtle managment.

Almost sure that probably 3 and 4 can give a extra minute.

Cheers,

Juan