How fast a 120 vs 240V EVSE will charge your FFE is not about its efficiency but about how quickly it transfers electricity into your vehicle's battery. Higher voltage is like higher pressure in a hose, and yields faster flow of the electrons into the car.
The EVSE itself, the box on your wall, is over 99% efficient because it contains only sensing and safety equipment; it doesn't process the electricity. Test results for various models at 3.3 kW charge rate (like a 2012 Leaf) are at http://avt.inel.gov/
, with models listed in the right bar.
The actual charging of the battery, converting your AC input into DC and conveying it into the battery, is all done by your car's onboard charging unit, which accepts electricity at a maximum rate of 6.6 kW. Assuming unity power factor (it's probably near unity and I'm trying to find out from Ford what it's actually rated at), 6.6 kW would be close to 6,600 volt-amperes (VA), which divided by 240 V means a maximum current of 27.5 A. Net of worst-case cable losses and other details, that corresponds to a nominal EVSE input of 32 A, delivered through a 40 A circuit breaker at your panel.
This helps explain why your FFE will do a full charge in just over 3 hours at 240 V but more like 18 hours at 120 V. And of course these charging speeds at different voltages (and constrained to your EVSE's maximum power rating, so only half as fast if you use a 3.3 as a 6.6 kW-rated EVSE), have NOTHING to do with your car's predicted range. That depends on how the car's predictive algorithm interprets your three Ts—Technique, Terrain, and Temperature—based on your past 20 charge cycles (or fewer if some variable is changing rapidly). This is discussed in the general owner's forum and elsewhere.