Had an interesting flight in a glider at Omarama a few years ago, with a local instructor. We got into the wave lift at about 7.000', then headed for Mt Cook doing 100 knots, with the VSI on 1,000 fpm up all the`way. Arrived over Mt Cook 12 mins later at 19,500', smooth air all the`way! Had to use the dive brakes to stop the climb. Great fun!
To answer the original question, the best speed to fly for max climb in the downdraft if you insist on staying in it is obviously best ROC speed. Much cleverer to fly out of it, the faster the better (beware possible turbulence!).