Sometimes you change a plan, other times you start off with a clean slate. In electric vehicle design these two approaches are used as well, you have most of the traditional car makers changing a current model, others are developping a totally new car.
An interesting article from my friends at Green & Energy on the topic is linked below, it is well worth a read on this design issue.
Purpose design vs. Conversion design
If you regard the Mini E in contrast to the SIM-LEI in terms of energy consumption you have about 18 kWh/100km for the Mini E and ca. 7,4 kWh/100km for the SIM LEI (both figures are rather optimistic). But this already shows what happend in a quite short time and I´m sure that there are still further potentials for reducing energy consumption of EV, which would also improve the economy of such cars.