I just finished reading Weaving of Mantra by Ryuichi Abe and it's concluding sections has prompted me to ask a question that I have pondered on for years now.
In the Koyasan Shingon school, how important is precise pronunciation of mantras considered to be?
In lay practice, it is clear that the transliterations of sanskrit mantras to the Japanese syllabary are only mild approximations of the Sanskrit pronunciation. To explain Sanskrit prior to the kana system, Chinese characters were adopted for their sound approximations, which do not appear to have done any better. However, it is apparent that priests of the Early Heien period were aware of these shortcomings because textbooks eventually began using diacritical marks to try and capture the linguistic qualities not found in Chinese or Japanese. In spite of these attempts, mantras today are recited only in the syllabary of Japanese. Or at least that is how I have seen it from the perspective of the lay community.
Some followers of Tibetan Esoteric Buddhism that I have spoken with are ademant that the precise Sanskrit pronunciation is very important. Given that Kukai taught that all letters are seed mantras originating from A and that all letters contain the entire teaching of Mahavairocana, I could be inclined to believe that the whole practices that are associated with the sound of a mantra are what give a mantra its efficacy rather than the specific sound itself. However, I have absolutely nothing from priest or scholar to confirm or deny such an idea.