Impurities in the PET can dick with high frequency response by changing how the diaphragm sympathetically resonates with the sound I personally feel, but if the manufacturer is already using high quality PET there's not a tremendous amount of added value, except maybe the value of changing the diaphragm tuning which to be fair can be a pretty big change in sound. The value of Maiku over 3U Stock is probably sonic differences from level of tension. You could certainly ask the manufacturer to do that in house, but I think Maiku Probably just has fun doing it this way, and I think that's a perfectly valid justification for doing it, And if people are willing to pay extra for someone being passionate about it I don't think that's a terrible thing. Kind of getting into the realm of art thoughI find it more believable that some of these large asian manufacturers would have more comprehensive setups to diaphragm their own capsules, than what is visible with some of these “third party skinner” methods.
While I can imagine some valid reasoning behind adding a diaphragm out of house, I can’t remember ever hearing what I would personally feel to be valid reasoning conveyed. Usually there is mention of steps I am more doubtful about.
It is a rough scenario though, because if any company gets more detailed about what they are doing it provides a checklist for someone else’s potential business model.
The only full blown business companha with a genuinely respectable amount of transparency is AA. They went through their entire development process, manufacturer selection process, etc with names on another forum. I always really respected them for that.