Yes, it is unsound. Quantum mechanics has features which aren't observable (the imaginary components of the wavefunction, the value of position and velocity at the same time, etc). However, this does not mean less computation, because in order to simulate the system you need to represent the whole wavefunction and everything that happens to it whether it's observable or not. Quantum mechanics is elegant in many ways, but low computational load is not one of them.

See Feynman on the subject: https://people.eecs.berkeley.edu/~christos/classics/Feynman....