Remember when you are calculating your total RPM to use 3.7v/cell with lipos. While they do have a max charge of 4.2v/cell when they are put under a load the voltage will drop. The nominal voltage is a good average to make your calculations with.

Also, the kv rating given on motors is unloaded, so that's what it will spin at when bench testing the motor not connected to any vehicle drivetrain. Once you put it in the vehicle the kv will drop a bit. Usually not big enough to be that noticable, but nonetheless it still occurs.