Dennis,
I don't think they actually lose power or energy, but what happens is that the internal resistance goes up. So now when you try to draw your 300 watts, the IR voltage drop in the battery eats some of the energy. So your ESC compensates by opening the throttle more, letting more current flow. The net energy into the ESC is the same (lower volts, higher amps), but the battery is absorbing more energy by heating up faster than it normally would. Of course when it warms up, its resistance goes down, so eventually you tend to get back. But the initial heating is lost to the ESC/motor combo.
Here is a spec sheet for my 2100 mAHr cell.
Notice the discharge is at 0.5C, much lower than we do. I am guessing this is so they can keep the battery at the specified temperature. In our case, the battery temperature is changing.