I think I read somewhere that "Armstrong was doing 38mph on the flats" in a TT once. At first I thought, that's a ridiculous exaggeration. With a tailwind perhaps, but c'mon. After reading this thread I can see it's possible. In an interview a long time ago, Hincapie said he'd been working on threshold intervals at 500w. Armstrong is probably around the same.
That blows my freaking mind.