Damien has been running the motor at ~40V iirc but obviously that has limited power. I suspect we don't know the power/voltage tradeoff at this time but Damien is the expert
Long answer:
Any electric motor has a power/voltage tradeoff. It tends to be rather linear. Half the voltage means you can only get to half the rotational speed. So, a motor meant for 380V will run but only be able to go half as many RPM if you run it at 190V. Now, there is also the question of possible torque. Torque is a function of current so as long as you can push the maximum current you will get the maximum torque. A motor which is spinning generates a back electromotive force (BEMF) which counters the forward voltage you are putting to the motor. Motor windings have a certain resistance. The end result is that you need a certain number of volts over the BEMF in order to drive a motor to full torque. Let's say this value is 10V just to make it easy and round. So, from a stop you need 10V of battery power to get full torque (Obviously in reality you need more than 10V to start but bear with me here). As the motor begins to spin it generates back EMF so lets say at 100RPM the BEMF is 5V. Now you need 15V to drive it to full torque. If you have 40V then you will quickly come to a point where the BEMF gets high enough to start to limit your available torque as you will no longer have the 10V voltage over top of BEMF. At this point you will hit an equilibrium where you get just enough torque to keep moving at a certain speed and everything is balanced. To go faster you'd need more voltage.
So, really the voltage you give the motor sets the upper limit for RPM and not a lot more. However, commercial controllers usually have a lower limit for voltage. This isn't for the controller necessarily. It has that setting so that you don't ruin your batteries as they do care if you run them too low.
One last detail that is of minor significance: The transistors in a motor controller "drop" voltage. That is, the voltage out is less than the voltage in. IGBTs have a fair amount of drop. The drop is related to current at least weakly. Thus, drawing a lot of current at lower voltage is pretty inefficient with IGBTs. This is partly why they use higher voltages in most cars. You then can wind it so you need higher voltage and lesser amperage. Then the drop for the IGBTs is minimized and a small chunk of the voltage. What I mean is, if you have a 2V drop at 40V then your effective voltage is 38/40 or 95% of the incoming voltage. If your voltage is 400V then you get 398/400 or 99.5% of the incoming voltage. Obviously the 400V is more efficient.
Short answer: Within reason you can run it at a lower voltage, just keep in mind that your top speed will suffer pretty much linearly with the drop in voltage. Half voltage = half speed.
