2 LPH is more like an anomaly not a gain i wouldnt even bother measuring it, i am not real familiar with the ER RS series but i imagine you would have better results at 6-7", shows what i know.
As far as i know the deeper the pump the higher the back pressure and lower the air, most people seem to keep their skimmers in 6" of water or slightly more.
i know if i had my ATB in 10" it would overflow the cup pretty easily because the output on the skimmer is so far underwater