I have modified the Bill of Materials and assembly steps to include information on how to use an aftermarket iPhone 5 battery (~1500 mAh) instead of the 500 mAh LiPo.

I have done an initial charge test and play test for about an hour. The battery never got hot on me so I'm assuming that all is well with this modification. Keep in mind that this hack uses an Adafruit PowerBoost charger with a battery that it wasn't designed for.