Electric motors do have a constant power region and its ideal to operate there. I would expect teams to modify their transmissions to better fit their motor as more parts of the car are opened up for modification.
The engines do have a fairly flat torque curve however, so you can get away with a constant speed reduction, which gives a decrease in weight (like the Tesla). One reason they might run a transmission is so that they constantly run the engine in the most energy efficient zone, giving them a longer battery life.
A flat torque curve means you get more power at higher revs. Power = torque x revs.
If torque is the same at 5k revs vs 10k (to pick numbers from thin air), when you downgear 10k 2:1, you double the effective torque over 5k at 1:1, for the same output revs.
It does not work like that. To run twice the speed, you run twice the voltage so you run half the amps. Torque scales almost linearly with amps. With the same power input, the power output is all about the motors limits versus your conversion efficiency.
The battery layout and chemistry will limit the actual power that can be delivered to the powertrain (discharge rate). A faster electric car is not one geared higher, but one which converts the most of this energy into motion.