You would be well advised to completely forget the concept "RMS power". While it can be defined mathematically, it has no physical significance whatsoever. It was a term cooked up by idiots in the hi-fi marketplace of the '60s. These guys simply did not understand the reason for existance of the concepts rmms voltage, rms current and _average_ power.
RMS voltage and current are, however, entirely meaningful. If you compare half- and full-wave _voltage_, you will find the rms voltage of the half wave to approx .707 times the rms voltage of the full wave. With a resistor load, Power=(voltage squared)/resistance thus when you square .707 (actually 1/SQRT(2)), you get 1/2 so the half wave rectified wave delivers exactly half the power to the load.
Ted