I read the above with interest as I, too, am just now working this piece up. Martha's performance is interesting, to say the least--she hits the agitato portions presto, and then slows down markedly for the lyric portions, and not just the B section in B major. She plays so clearly, and with such control, that it works. Glenn Gould's approach, on the other hand, is very Baroque in approach, he pretty much sets a tempo and sticks to it.
Tempo--IMHO, should be a personal decision, based on many factors, including the instrument at hand. Even if the composer writes an exact metronome tempo (e.g., Beethoven, Hammerklavier), this is to be taken with a large grain of salt. There are stories of Beethoven changing metronome markings on the same work. Do what works for you and your interpretation.
Best of luck with this great Brahms!