Couple years back I fondled a real-life Bf-109E, and played with the leading edge slats. They truly move with almost no pressure, but even those were faulted for asymetrical response in use.
Getting a leading edge slat on a model, with their much lighter (proportionately) mass and the significantly lower pressure it will see to operate consistently on -both- sides... that's the problem with "devices". 2 servos makes the plane heavier than a plane without the 2 servos, everything else the same.
Trailing edge devices OTOH work well and are the way to go, because they don't have to move (translate) fore-or-aft and the servo(s) are already there.