the ineffiency of charging at 120V vs 240V is mainly due to fixed losses associated with the charging process, not the circuits themselves. Because 120V charging takes much longer, the car is kept awake longer, and as evidenced by the enormous threads on vampire drain, an awake vehicle drains the battery much faster than an asleep one. Charging at any speed means that same awake draining force is being applied. The faster you complete your charge, the less loss from "awake car" there is. In cold weather, 120V charging sometimes manages to only break even with the increased inefficiency due to temps. This was a bigger problem with S and X due to their dedicated resistance heater for the battery.
On the other end of the spectrum, if you're on a 240V circuit and charging at high amps, you increase resistance in the wire, which is loss via heat. You can see the effect of this by lowering your charging amperage and watch your voltage go up a bit. A thicker gauge wire can minimize this, but the wire will cost more $$.