It would have to be very thick to house that battery. Honestly I wouldn't want the iPhone 6 any thicker than the camera lens. Had they made it flush all my design gripes would be solved.
The extra bit of battery life probably would have been appreciated.
That does make me wonder, how many Mah of battery does it take to make an appreciable difference? I'm gonna imagine that 100 Mah might not be noticeable but that, say, 500 would be.And wouldn't have made a difference.