[schwarze-man]

The leading edge "stall strips" should perhaps be called stall warning strips as they are usually used for that reason. At higher angles of attack the sharp point of the strip makes an increasingly strong disturbed stream of airflow rearwards. In cases where the horizontal tail surfaces fall within that disturbed flow, this technique can be used so that the tail surface is vibrated by the turbulent airflow and the vibration is felt by the pilot through the controls.
Some aircraft will show satisfactory warning without this method. Overall, the improvement is purely in the aerodynamic warning of high angle of attack, not the performance.
The presence of the strip does usually make a small reduction in the aerodynamic performance of the wing in normal flight.
Refering to Bf109 l/e slats, There will be a small reduction in efficiency due to the slight effect of the slat spoiling the wing shape. However, the stowed position is very clean and, when the slat deploys itself at high AoA the performance of the outer wing is greatly improved.
Washout on wings is not a perfect panacea, there may be adverse effects, including increased zero-lift drag and drag at high speed.
Overall, these points are generalised and individual aircraft will have their own characteristics.
Cheers

SM