I think you probably care more about voltage sensitivity than power, but you can compute one from the other, for a headphone of given impedance.
Amir's review showed the Atom delivering 0.25 W into 300 ohms at high gain (no clipping). That's 29 mA of current and 8.6 V. If he used a 2V input for the test, the max gain ratio was 2:8.6 which is 20*log(8.6/2) = +12.6 dB.
If the input voltage were only 1V, that's cut in half, which means the output voltage would be cut in half, which is 4.3 V. This is enough voltage to drive Sennheiser HD600 beyond hearing-damaging levels (115 dB SPL).
Simlar calculations suggest that with a 30 ohm load, the max Atom voltage output is 6 V. That is, the Atom is current limited. Half of this is 3 V which still drives an HD600 to about 112 dB.
In short, it looks like the Atom has enough gain to drive low voltage sensitivity headphones to ear-splitting levels, even if the input is limited to 1.0 V.
Half the voltage is 1/4 the power (into the same load). So for an input limited to 1.0 V, take Amir's power measurements and divide them by 4. That would be 250/4 = 63 mW into 300 ohm and 1100/4 = 275 mW into 33 ohm.
PS: I just noticed that my calculations, which I backed into from Amir's measurements, are almost the same as the figures
@NTK posted above from the Atom specs published by JDS.