RE: lipo question for the experts
It sounds as if you already have some lead weight in your plane for adding weight reasons?
If so, adding a heavier battery with twice as high as a mah and removing some lead will basically double your flight time.
All things considered, if your total plane weight stays the same BUT your mah goes from 2100 to 4200, you'll be doubling your batteries capacity and not increasing the draw on it.