Until Boeing changes their manual a clean wing is your best L/D for gliding.
Leading edge devices have a huge reduction in stall speed. The first notch of flaps is typically about 50% of the total stall reduction you get from flap extension.
On what airplane?
On the 747 there is a 20 KT reduction in stall speed (or maneuvering speed, however that may technically be defined) for each of the first 3 "notches" of flap -- 1, 5, and 10. There is another 20 KT for the last 2 increments (through 25 to 30). While the 747 Classic and 747-400 have completely different wings and LE/TE flap extension profiles (e.g., the Classic has some TE extension at Flaps 1; the 744 does not), the speed reduction increments are retained.