Higher voltage means less current for the same power. Since the power dissipated on the battery is proportional with the current you want to draw less current. What a lot of people do not understand is that the lack of a gearbox in an electric car is a big problem. When the motor is running suboptimally the excess energy taken from the battery will be transformed in heat.
It depends what speeds make the motor run sub-optimally for a particular vehicle. The range 0-80 MPH is sufficient for almost all normal driving. If you want to make an efficient track car that can operate at high speeds optimally by all means introduce a gearbox. Its going to add more weight, complexity and maintenance though. I don’t think thats the target for any of Tesla’s cars besides the 2020 roadster though.
For single-speed EV’s, the motors are operating very far away from their maximum efficiency point at high speeds. By upshifting, you can reduce the number of revs required at top speed, which should improve efficiency and reduce heating of the motors.
Alone it's probably not enough. In combination with other design decisions, it could be the key though. Take for example the smaller battery. Normally that would be a big limiting factor as battery size is somewhat indicative of maximum power output. But by shifting gears once through the acceleration curve, they can compensate for the lower max power. Furthermore, they could keep higher speeds with less constant power output.
The reason this matters is because a smaller battery will produce less heat overall, which will be easier to cool, and so on.
Anyway it's just a theory. The facts remain: the Taycan can do something the Teslas can't and this gearbox is the biggest single meaningful difference in design of the powertrain. It stands to reason they are related facts to some degree or another.
Most electric motors are most efficient and can generate the most power within a certain RPM range. (It's a lot wider than gas engines, but beyond a certain speed the torque falls off.) If you can drive at high speeds at a more efficient motor RPM, then that's less excess heat for the motor and inverter to deal with, and less current draw from the batteries.
Electric motors are pretty much opposite of gasoline engines. Motors have peak torque at 0-2krpm and it drops off rapidly after that. Gasoline cars tend to hit peak torque near the end of their rpm range (ignoring turbos that can change that in any number of ways).
From what I can see, most electric motors are tuned to have peak efficiency at ~5krpm which is conveniently around freeway driving speeds.
