Seems like a pretty accurate estimation, to me. The phone itself is 7.3mm thick, which means the case would be at least 1+mm on each side. Let's be generous an

Seems like a pretty accurate estimation, to me. The phone itself is 7.3mm thick, which means the case would be at least 1+mm on each side. Let's be generous and say the battery's 3mm thick now. Adding another 1mm would be a 33% capacity improvement.