A few things.
Since some of the cast recorded their lines "on the go" at whatever studio was convenient, I doubt there was much "direction" at all. I've directed professional voice actors (from Janet Waldo to Billy West) in studio and there's an art to getting them "into the moment" so that they're ACTING and not just reading their lines. Even the pros get curiously flat if you don't work with them on this. Since the Trek
cast weren't experienced voice actors anyway, it's no wonder their performances were often so meh, considering the circumstances.
We've discussed this before and there's some question as to if the voice tracks even still exist separate of being mixed with music and sound effects, so reusing any of the sound would potentially be very difficult.
re TOS-R type "improvements", "better" IS debatable, despite Rama's assertion otherwise. In a film everything must be seen in context. I could cut Star Wars VFX (with the color saturation turned off) into old Flash Gordon serials and while the effects themselves would arguably be of better quality, seen as a piece they would not fit with the surrounding period photography and stick out like a sore thumb, which is often the case in TOS-R. Is that "better"? You decide.