the impedence of the headphone has no effect on the battery life.
what matters is the Sensitivity rating (measured in dB@1mW). the higher the sensitivity, the lower the power draw for a given volume.
The power drawn is found by Voltage X Current (well, there are other factors in Ac audio, but they mean very little here)
Lower impedence headphones draw more current, but you'll end up playing them at a lower voltage (ie volume control turned lower). Meanwhile high impedence phones draw very low current, but they need more voltage (turn up the volume control). If you measured voltage and current for both high impedence and low impedence headphones, and multiplied these two together, you should end up with the same power if they have the same sensitivity and were playing at the same volume.
A potential problem with the RA1 is that it was designed for low impedence headphones, meaning that it can put out lots of current but not very high voltages. Even at full blast, it is probably putting out less voltage than an amp designed for Senns. As a result, there could be a problem reaching high listening levels if you used Senns with the RA1. But other reviewers have stated that they can get enough volume, so i guess this is not a problem (of course different people listen at VERY different volume levels, so this may not apply to you)