I was watching Kioku no Mori from YKL#16 again - jumping to YouTube from the YKL#21 thread here - and while I was enjoying it, I saw some old comment threads that led me back to watching Miezaru Ude again!. In comments back then, I really felt then that Kaori's voice was peak and I still consider it one of her best overall vocal performances, but I am realizing even more how a massive part of what I enjoyed was Revo's unconventional composition for her that enabled her to sing across her range and be showcased to that great level.
It's the question again of how IMPORTANT the composer and music is to hearing a singer at their best. Just singing average ballads or generic pop songs or covers with basic musical progressions (no matter how emotional the vocal performance) can only bring out an average performance from a singer, no matter how much potential they have or their tonal qualities.
Of course, during the Miezaru Ude timeline, Kaori was still quite young, barely out of teens, and so her voice was still developing. There was a lightness and softness to it that was endearing. She also used her falsetto quite a bit in places where her older self could use a mix voice.
As Kaori matured, her voice got stronger and heavier (more dense, fuller-toned), and also much more expressive (with greater singing experience too), sometimes losing the straight coolness and consistency of the past. I think it's important to be able to restrain yourself and approach softness even when you've gotten more power, vocally. Of course, when your voice changes there's less you can do to replicate your younger voice.
I mean, Miezaru Ude Kaori couldn't deliver the chills I feel from the power of the YKL#16 Kioku no Mori Kaori (set up beautifully right after Yuriko's masterful bridge). The younger Kaori would have brought a different feel to the performance (maybe closer to Yuuka's original, actually). I am also curious how the current Kaori would take Miezaru Ude if she was asked to sing it again by Revo.
I have to say, I have preferred Revo's use of Kaori's voice overall to Kajiura's. I say overall, because I enjoyed Kaori's Fake Wings from YKL#2 (of course her voice was still not far from Miezaru Ude), so it's maybe the softness and delicacy of her younger voice I liked. Maybe it's the qualities of Kaori's early vocal that were just better showcased by Revo's music for her. If Revo was to write for her voice now, he would probably write in a different way.