I am not sure I understand the objection (or maybe there wasn't one?) but since there seems to be some confusion in this thread on this point, the electrical ef

I am not sure I understand the objection (or maybe there wasn't one?) but since there seems to be some confusion in this thread on this point, the electrical efficiency of electric thrusters is very well defined, it is: η = ½Q·(Isp·g₀)² / P where Q is the mass flow rate (kg/s) and P is the input electrical power. Hall thrusters have efficiencies typically ranging from 30% (small HTs) to 60% (large HTs). Gridded ion thrusters have slightly better efficiencies (add 5-10%), mainly because they operate at higher Isp (they would actually have worse efficiency than Hall thrusters when operating in the 1000-1500s range). Why I object to the (admittedly common) characterization of Isp as an efficiency: it is always better to have a high efficiency, whereas Isp is always a trade-off between mass budget and power budget so optimal Isp is always mission-dependent.