Thred creep, but:-
LKMflyboy. XO and XN might be heavier with their thrust reversers, but that is only by 300kg or so. I flew XO this morning, and it burnt no more fuel than any other aircraft we operate. On the short sectors we fly, more fuel is wasted due to the poor ATC route structure into the Scottish TMA forcing us down early from cruise altitude - eg FL260 by Margo or NEW and FL70 by Tartn.
O and N are both "MP" versions, and do have the advantage of a higher MZFW and a higher MLW.
Ape