[gardano]

I'd argue that in baroque music, much of the notation is a scaffold upon which to build the performance.

Often it's forgotten that articulation, ornamentation, and improvisation are all supposed to be assumed in baroque performance practice. And that in many cases (nowadays), all three of the above are tossed aside (thankfully, less so nowadays -- with performers skilled in performance practice).

Remember: Bach was much better known as a performer/improviser rather than as a composer in his own days.

Combining the complexity of Bach's compositions with the complexity of his (or someone else's) actual performance just adds additional layers of the type of sophistication that you're speaking about.