Headphones impedance and the SPL
The only spec that matters is sensitivity.  For normal speakers, sensitivity is usually listed as dB Per Watt at 1 meter.  For headphones, it is usually listed as db per milliwatt at "ear distance".

There are lots of things that can affect sensitivity, and impedance is just one of those.   

What I can say is that at the same "volume setting", one of these headphones will consume about 7.8 times more wattage than the other.  But how that impacts volume is completely up in the air because we don't know the sensitivity.