Optimal headphone impedance
Looking at the headphones as a simple resistor, and the sound source as a simple voltage, it is quite plain using Ohms Law that yes, that is true.

If the headphones are 8Ω and the sound source is 1.2V (for example), the current would be \$I=\frac{V}{R}\$, which is 150mA, and the power (\$P=\frac{V^2}{R}\$) would be 180mW.

If the headphones were 4Ω the results of the same formulae would be 300mA and 360mW.

So yes, the lower the impedance the higher (in basic terms) the power, but also the more strain on the amplifier.