The winglet acting like a sail is not what we are talking about.
WINGLETS DO NOT PRODUCE "THRUST" IN ANY WAY SHAPE OR FORM. They produce lift and drag reduction is all such that your L/D is better for the given airplane. You can attribute a thrust reduction to the installation of winglets allowing you to fly at a lower thrust to maintain speed without the winlgets, but the winglets are not "thrusting" the airplane to make up for pulling back the throttle.