Check the Bicycle Quartely web site for drag; I think they have their dynamo comparison article up.

My bottom line is not to worry about the drag. 1 W (lights off) extra for the Shimano over the SON is not a big deal. I usually turn the lights off for a winter day ride, but I've left them on over the nearby ridge and a good distance further before I've noticed the funny shine on my fender.

Somebody compared the drag of a dyno hub to a standard hub and concluded it amounted to a 10 foot rise every mile of the ride. IIRC their conclusion was, "If that's a hill, I'm a Dutchman!"