I currently get 4.5 mph off a NEMA 5-20. However, that's at an abysmally low efficiency of 79%. If the 3rd gen battery/charger can solve these efficiency issues, then you can get an appreciable charge rate. My napkin math shows if Tesla can achieve 95% efficiency with 215Wh/mi then the charge rate will be ~8 mph. There is another reason why this is important, because with the Model 3 release Tesla will be adding significant amounts of battery storage to the grid. It makes sense for this battery storage to be plugged in during sunlight hours, when people are at work. Installing NEMA 5-20's for many parking spots is much easier and cheaper than installing EVSE's everywhere.