Re: Would TMP have been better had they used the TOS uniforms?
On a practical level, it also bears noting that in the movie era, the actors were starting to lose their physiques. Doohan was the first, who seemed to put on a lot of weight between TMP and Khan, followed by Nichelle Nichols and Shatner. The woolen red uniforms helped to disguise this better. There was no way anything resembling the TMP uniforms could have been carried all the way through to Trek VI without the entire Trek cast being ordered into the gym.
Indeed Shatner says he ran and worked out frantically for TMP. Time well spent...
Soon oh soon the light, Pass within and soothe this endless night, And wait here for you, Our reason to be here...