I have to say I have not found myself that 'the more we know about how music was performed when it was new, the better insight we have into that music. '
The inaccuracy of that statement is borne out by Ian's post above. It may not affect how the work is performed today, but knowing about the circumstances of its inception certainly aids "insight" I would have thought.
I think the great performances of Mozart and Beethoven (and Schubert's piano sonatas) began in the 20th , when musicians had the knowledge and the instruments to do them justice
20th century performances may be
different, but what's your evidence for denying that "great performances" of these works didn't happen earlier?
when the composers wrote, they were ahead of their time. .
Sorry, I don't buy that at all! No composer is "ahead of his time".