I have attached an EXCEL file that should help those wanting to know approximately what the effect of a larger battery will have on their "airtime". This calculator can be used several ways:
You can put in the mAH and voltage of your current battery (use 11.5V for 3-cell and 15V for 4-cell) and change your "inefficiency" number up or down until you get the airtime you have measured on your present setup.
I call this number the "inefficiency" because as the number goes up, the efficiency goes down. So, since it represents 1/efficiency, I call it IN-efficiency..
Once the inefficiency number for your craft has been determined, figure out what your craft will weigh with the new battery, change the maH number and enter those into the appropriate cells. The airtime number should be very close to the new airtime you can expect with the new battery.
Keep in mind that if your motors or props are heavily loaded, the inefficiency goes up rapidly, and the approximation will no longer be valid.