Yes increasing the mAh rating of the batteries SAFELY increases the flight time. In your case you are looking at a 1/3 increase in time over what you have now.

You didn't mention the weight of the plane, but if 1/2oz is less than 1/20th the weight of the plane, you will not notice much difference because of the weight.

LiPo is the way to go though as you get more power at a lesser weight than what you have now.