Just regarding the motor alone, you could theoretically put voltage up to the coil insulation max, which is usually something like 400vdc or so. But the driver input is probably more like 30-40v max to start with.
The torque simply does not care which voltage is it, only current. But there is a dependency between them in the sense that the voltage across the coil builds the current rise, and the problem arises when the voltage is so low that current does not rise fast enough - thats when voltage matters, but that is because voltage is too small. If the voltage is "too high", it does not matter because the current reaches the set peak after which voltage is interrupted. So a bigger voltage does not do anything, except it gets the current to "erect" faster thats all.
The torque simply does not care which voltage is it, only current. But there is a dependency between them in the sense that the voltage across the coil builds the current rise, and the problem arises when the voltage is so low that current does not rise fast enough - thats when voltage matters, but that is because voltage is too small. If the voltage is "too high", it does not matter because the current reaches the set peak after which voltage is interrupted. So a bigger voltage does not do anything, except it gets the current to "erect" faster thats all.