I'll spoil one result for all of you upfront: the Sanyo Denki San Ace 1011 is king of the hill. And by hill I mean universe. It's really, really good. In addition to its insane empirical results (see below), it makes NO extraneous noises and undervolts like a champ. I have five and they all perform like this and they all start at 4V or lower. They could kick Chuck Norris's ass....silently.
They're ball bearings as well, so you don't have to worry about the sleeve bearing inconvenience. They also perform past their listed specs--something that's exceedingly rare in these SilenX days. Good luck finding any though...and when you do, they're $25 a pop.