Hi
Just my 50 cents on this mod. Higher air resistance. Extra stress on components due to extra weight. More down push from propeller wash. Higher EMF fields through the craft (DANGER of compass error). Reduced agility. Reduced "echo" speed. I meant the speed at which you can travel without stressing the batteries. Extra noise.
So I named many minuses against one plus, some extra flying time.
If I were to insist with this mod, I would first make sure my cables to motors are twisted and I will use extra copper tape (NOT ALUMINIUM) as EMF screening and definitely I would use the low noise propellers. (they spin slower for the same output).
Unfortunately on the Mavic, you can't throw overboard to much. "OcuSync" 2 extra range comes mostly, if not totally, from higher currents feed to transmitters and better cooling compared to Phantom 3. There is a fan kicking in sometimes even in the remote I think. All the metal plate underneath (20 grams if I remember well) is a heat sink so please don't think of getting rid of that one!. The only safe weight reduction program is the wifi card, which nobody uses, about 10 grams. As well you can get rid of 30 grams approx by dumping the battery shell and using a cable tie or rubber band to secure the battery in place but this is not much.
I have had 2 Mavic pro's lost "in action" due to battery mods resulting in strange behaviours as exit Litchi mission and hovering in place(not ATTI flyaway), and auto-landings with no apparent reason. It was my own stupidity to insist until only the remotes were left ...Both were at about 960 grams using a stock second battery with no electronics backpack mode.
So I went back to good old P3 because there is a lot of fat you can trim.
I have for over 2 years a double stock battery P3 weighing in at (after the last extreme slim down) "only" 1314 grams. That's 30 grams over a stock Pro. 43 minutes hovering to 15 percent battery with stock propellers (22-22 degrees Celsius), 300 meters above sea level), over 1000km flight distance so far with no issues, and the same flight characteristics as a stock one. The main "design" feature is that the bottom part of the shell is made up of the second battery.
I don't say what you do is wrong and what I do is right, I just wanted to share my experience for others which consider battery mods.