well mpge is kinda silly to begin with (I mean really silly), but also different charging methods have different efficiencies as well. But with a 240v supply, it is in the 90% efficient range. Any comparative perspective on it will be kinda arbitrary, you can do well to wheels, you can do costs (including taxes/subsidies), or "environmental" costs, you can pretend the charger is the "gas pump" and ignore losses there (like you do in the gasoline supply line), you can look at kwh, there are a lot of ways to look at it. I have no idea what the EPA is doing, and I probably wouldn't like it if I did, since they are going to try to dumb it down with some rediculous unit like mpge.
But for the most part I'm interested in the efficiency of the vehicle, not necessarily the charger, because the vehicle efficiency determines range and performance. And you can fudge in %10 for charging overhead if necessary for cost comparisons. There are more efficient chargers, but they probably won't pay for themselves in a resonable amount of time. I expect they will become commonplace eventually.
here are some leaf specifics, I think most folks are going to go with 240v charging, or at least they should. (and short of a complete diy, the openevse kit looks like the best bargain currently)
http://www.mynissanleaf.com/viewtopic.php?f=31&t=8583