I think that retractable leading edge flaps would be a tremendous amount of work to make, but carefully shaped fixed slats would not be too much bother, and would increase lift coefficient by at least 40%; less than most retractable slats, but a fixed slotted wing normally produces much less drag than a wing with extended slats. It is best to bear in mind that anything that increases lift coefficient of a wing usually also results in a more abrupt stall, and that high lift coefficients mean high induced drag. Increasing lift coefficient by 40%, for example, will nearly double induced drag at near-stall airspeed, and at the decreased airspeed made possible by the higher lift coefficient, the tail feathers will be much less effective, particularly the vertical tail in resisting yaw that may lead to a snap roll into the ground.