The flap setting in relation to maximum landing weight differences was due to a Landing Climb Requirement. '3.2 gross gradient. U/C down, landing flap, all engines at max thrust and speed greater than 1.3vs with overshoot capability from the threshold'. That quote came directly from my old course notes. I do not think it was restricted to just the LR as we operated straight 200's as well. The LR was 89358 TOW compared to the straight 200 at 86409. Both had the same thrust from JT8D-15 engines. So naturally, close to the ground at high weights on a hot day, the LR was not very spectacular. Once it got away from mother earth it went like a rocket.
I have also learnt something new, with some operators landing with flap 25. I can not work out why you would do that? Save fuel and less noise maybe? Flap 25 at Hamilton Island would guarantee a swim back to the terminal.