Re: Less drag, not more lift
You have it. Without an endplate of some sort, high pressure air below the wing spills around the end and equalises pressure, meaning that the last couple of feet of a naked wing contribute nothing to lift (but still to drag).
By preventing spillover, this air forms a neat vortex. Drag is reduced and lift improved. They serve the same purpose as the five large feathers that stick out from the end of an eagle's wing, which is where the A380 design team got the idea from (and as they came up with the system and are adamant it contributes to lift........!).
A necessity for the 380 to keep the wingspan down to acceptable levels and retrofitted to other aircraft as more lift and reduced drag reduces fuel burn on takeoff and climb.
Maybe when retrofitted to an older wing design it merely serves to reduce drag but when coupled with a wing surface designed to get the most out of the system...................?