Exactly the point, at some point allowing the aircraft to slow to 108 kts probably resulted in worse performance than maintaining Vref Flaps 30.
Quoting Boeing's recommendations probably has to be clarified or expanded. I doubt they recommend maintaining Flaps 30 if you're at 2000' AGL. At some point there's a tradeoff. Again, that's experience, experimenting in the sim, and a smattering of luck if you'd ever have to make that decision quickly.
Everyone's familiar with the drag increase at greater than Flaps 20. Getting to mid slats is key.
You can verify stall information for your a/c's stall speeds. First notch of leading edge slats typically is 50-60% of the total stall speed reduction. Additional flaps have a lot of drag vs. amount of stall speed reduction.