rats,
Just goes to show different operaters have different "Standard Boeing" procedures!!
There is no recomendation in our FCOM, QRH or FCTM regarding F20 v F30 S/E, just the choice, with the "Performance permitting" caveat.
Yoy are quite correct regarding the Overweight C/L and this seems to answer Hullaballoos question, not withstanding Fullwings excellent advice above. I find it odd, however, that as MLW is a Structural limit, the c/l should use it as a dividing line for performance considerations.
You are, after all, not guarenteed to make climb gradient at weights below MLW, but depending on altitude/temp/varient will very often easily make them at weights considerably in excess of MLW.
Still, that's what the book sayeth...