What's really needed is some kind of overdrive unit or second gear. A programmable CVT would really solve all the performance and economy problems, but that would mean redesigning the whole drivetrain.
This is the approach the Volt uses. It uses two motors connected to a planetary gear set and will bring the RPMs of the main motor down at cruising speeds. (Since the motors are not the same size, maximum power is produced with just the main motor, with a loss of efficiency of course.)
One trick that drag racers have used for years is a dual motor setup, but with both motors tied together on the same shaft. Same simple gear reduction like a Leaf, no extra parts.
[The Leaf motor is not really over-revving, it's entirely safe at the speeds that car will drive at. What's happening is that electric motors become less efficient at higher RPM's do to the back-EMF they generate. They actually turn into generators, and that generated voltage is "fighting back" against going any faster. So efficiency drops as RPM increases.]
The trick with two Siamese motors is that you can run them in series at low speeds and switch to parallel for higher speeds. It's a little like having a two-speed trans without any extra moving parts. This really works great on a drag strip, I think the same idea could be used to lower the high speed back-EMF, and thus improve efficiency.