And remember, when it comes to battery capacity, weight and size, there's a point of diminishing returns.
Because my Phantom 1 was set up in a way that I could use any LiPo battery I wished, it was interesting to see where the weight:capacity ratio returns started to diminish. If I recall, 5400mAh was top of the efficiency bell curve - the "sweet spot" if you will.
The original Phantom series operates on a 3S system. But I took a calculated (well researched) risk on a 4S upgrade. Well wouldn't you know that that not only worked, but my little P1 saw a huge increase in performance and flight time, but I digress...
I guess my point is that, thought the Inspire 1 is a 6S device, I would be curious to see if the electronics could handle 7S or 8S voltage. Of course that kind of modification would be a huge undertaking.
Also worth noting, the heavier any bird gets, the less "crisp" the response to control commands it becomes, and more sensitive it becomes to high winds. So, depending on your usage, that's a factor, as well.
Good luck!
D