Thanks to Mimi Ezust for the link to this article.
(http://www.nybooks.com/articles/18400)

I was much entertained by a couple of Rosen's stories:

>> "Why is he playing it at that tempo," I asked a friend of Hofmann's with
>> whom I was sitting, and he replied, "He can't play it any faster."[5])

>> Leonid Hambro, who should be famous for having discovered how to play
>> Bartok's energetic glissandos on the black keys without stripping the
>> skin off his fingers-he used his wallet.)

and gratified by what he says about composers' instructions about metronome
markings;  he uses Carter as an example:

>> Some years ago, before performing Elliott Carter's piano concerto,
>> I played a few passages for the composer, and asked him if they
>> were fast enough. "They sound almost too fast," he replied. "I've
>> reduced your metronome marks by 20 percent throughout," I remarked.

What follows is really worth reading too.

Jim Tobin