I'll be honest, I have never noticed Leipheimer in races before Burgos 2001.Menchov got all his major victories after Rabobank fired Rasmussen and got a Carte blanche! He's a crystal clear overachiever and probably never was more than a mediocre rider in first instance.
Leipheimer his predessor at Rabobank on the other hand might be a huge underarchiver. Entered both the 2006 Tour de France & 2009 Giro d'Italia as main-favorite on paper. Won the 2008 Vuelta a Espana based on mere racing time. He never believed in his own strength. Riding extremely conservative and becoming a notorious wheelsucker. He was a gifted climber actually and even had the ability to make attacks. His test balloons for Contador at that Vuelta all were pretty shaky. It seems he just never knew when to make his move when racing in Europe. That's what has been so frustrating about him: He didn't dare to attack while he perfectly could have done so.
If Menchov was mediocre, then Leipheimer was abysmal.