You can't compare gas or fuel driven engines or cars with extended or extra gas tanks with a battery operated device. Your comparing apples to Mary Poppins.
If I have a 20 gallon fuel tank on my truck, and add another 20 gallon fuel tank I will get twice the distance with no fuel economy penalties. This is mainly due to gas/diesel vehicles are designed to carry weight. Either people or cargo.
Aircraft are totally different, it's all about weight, lift, and size.
Mavic Aircraft can carry (Some) extra weight, be it extra battery cells, release system, parachute, ect... This is why aircraft is built mission specific...
It's always a little disappointing reading untrue statements just because the internet says so. The people that logically/mathematically/determination, or experimentation are the pioneers of future designs. Simply stating that someone shouldn't do battery mods because they will blow up in the mahmoo dogface banana patch is not a relevant or even true argument. Instead when an OP ask a question for help we should stay on topic and provide ideas on how... Not flame their ideas.