I'm not sure that this is true.

http://fwd.five.tv/fifth-gear/videos...rsche-shootout

Though the linked-to video is of a Cayman, and the driver is a world class racer, he was actually faster with a manual than with a PDK.

 

Nice video. He may be a world class racer, but he certainly wasn't used to the PDK so I don't think that's a valid comparison. At one point he even says he accidentally pulled back instead of pushing forward. I don't think this is enough to draw the conclusion.