Richard Goode, Murray Perahia, and many other pianists in the top echelon routinely vary the tempos in Beethoven and Schubert. Listen to Perahia's Wanderer Fantasie--in the third movement, the tempo is anywhere between 66 and 88 at any given point. They do it so smoothly, skillfully, and judiciously that it's REALLY hard to notice--most people are totally unaware.
that is a true artform there. Really adds beauty to the piece.