NACA has tested this too. On their report "Effect of wing leading-edge slots on the spin and recovery characteristics of airplanes" (you'll wind it from naca site, search with keywords)conclusion in shortly was :
"For single engine designs with mass distributed heavily along the wings and for multiengine designs, open slots will have an adverse effect on spin recoveries. For single-engine designs heavily loaded along the fuselage, open slots will generally improve spin recoveries when the elevator is neutral or down; there will be only little effect when the elevator is full up"
Report contains much more on this.Check it out.
