Yes, certain Kiai are supposed to have certain effects (different effects for different Kiai) on the nervous system. Certainly, under the same theory, that would work the other way as well, as a conditioning method.
OTOH, I'm not sure whether it's the specific syllables, the specific intent, or the specific delivery that produces the effect. Would nonsense syllables produce the same effect if you believed that they would? Or is the human body biologically "tuned" to certain auditory stimuli?
O-Sensei came from a certain background, with certain beliefs, so some things would affect him in different ways (I expect) from somebody raised in a different manner. Or maybe not - it would be interesting if we could really put it to the test on a blind group...
It wasn't presented to me in the context of mental effects, but of assisting the solidity of the trunk in explosive movement. The idea was the abdominal cavity. The vocal apparatus helps manipulate pressure of the abdomen, through the diaphragm, by putting downward pressure on it, which when combined with pressure from below and the sides, surrounds it with pressure. In the transition between sounds, you can manipulate this into becoming a transition from a pressured state to an even more highly pressured state, and then, why not even the reverse? This is why the end sounds were unimportant, because firstly the goal was not the noise, and secondly because that would ultimately leave the diaphragm locked up and rigid. But even then, this was one limited effect, whereas Moriheu Ueshiba seemed to have a more extensive vocabulary of letters here than two, so I wonder then what he was doing with them or what shapes were implied?