When are winglets most effective?
At low speeds.  This is why you see them on jet airliners, which cruise a low "indicated" airspeeds, just low enough that the vortice strength is enough to get a decent lift vector out of the winglet from the flow.  An airliner cruising at M.78 say, at FL380, is only going about 2x its flaps up stall speed.  A straight wing airplane at 20000 ft cruising at 250 kt might be going 2 1/2 to 3x its flaps up stall.  The vortices aren't strong enough in that case and you see them on straight wing airplanes rarely, and if you do it'll be to improve rate of climb more than anything, or maybe simply "styling".