suitcaseman - you're losing me. They were at Vref -17/18 kts. Yes, reducing flaps decreases drag. It also decreases lift. If they really wanted to decrease drag Flaps 20 (mid range slats) is a large drag reduction.
You mention engines, etc, etc. In this example they were a glider. Power was low and fixed.
I don't know, and I doubt you know since you havn't published the numbers, if reducing flaps to 25 from 30 was a net improvement in performance in this case. I tend to doubt it and think that maintaining airspeed was more important.
But we've had months to consider this and still don't know the answer. The crew had seconds.