He would have done a fantastic job as usual and built upon his themes and material from the first movie.its just hard to imagine considering how great Horners score was.
Would Horner have done as great a job on TMP ? I dont think so.Horner had the benefit of a great movie on his hands with TWOK.Goldsmiths job on TMP was a lot harder as it fell to him to tell much of the films story during those long dialogue free stretches.He really saved that film and created a musical masterpiece.
I think he would've excelled on the major sequences in the beginning of the film like revealing the Enterprise, leaving dry-dock and the Klingon Battle. I'm not sure how he would've handled much of the score once they are traveling thru V'ger.
Nobody has a clue how great a Goldsmith TWOK soundtrack would have been. We know how great the TMP score was. We know how great the Horner TWOK score was. Why can't we just say that we have two great scores from two great composers and leave it at that?
I sorta hate this "what woulda happened if" questions. The answer is nobody knows.