I'm glad you pointed that.
In fact, I have studied historically oriented performance of baroque music lately. What you call "expansion and contraction" of tempo, in fact are tenutos for "good" notes and inegalité. These are described in several old music treatises, such as those of Couperin and Quantz (I don't know if CPE Bach talks about them in his essay, because I haven't read it completely).
I have read the Badura-Skoda book on Bach keyboard performance, and I have studied it with two teachers in college. In this second semester, I'll have classes with Judy Tarling (she's coming to Brazil in September!).
Unfortunately, pianists don't care about playing baroque music in an authentic way. After studying Badura-Skoda's book, I discovered that not even Tureck, Gould, Schiff of Perahia are true to how this music was conceived. Though harpsichordists ALWAYS play with much rubato! Baroque music has much more rubato than the romantic one, though these two are very different: in baroque, you only hold the "good" notes. In romantic music, you can hold the "bad" (weak) ones.
The only two pianists who play in a historically oriented way on the piano are Badura Skoda and Wolfgang Rubsam (he's famous for being an organist).
This is the Badura-Skoda book I have read:http://www.amazon.com/Interpreting-Bach ... skoda+bach
It's not only about good notes and inégalité, but also about ornaments and articulation. Articulation is the most difficult of all, in my opinion, since Bach didn't write any on his keyboard music..