The official ETOPs OE speed will be the absolute maximum TAS the aeroplane can manage in order to maximise the theoretical 180 min range. It will have no connection with long range cruise or engine out cruise, more with the absolute maximum capability of the aeroplane at drift down altitude. Expect an extraordinary number! The Captain still has the option of deciding if he wants to use that speed or cruise at a more sensible long range OE speed which would take the distance to more than 180 minutes on the day. But we are looking at an academic range only here, not necessarily a practical one!