tdracer
Its a little different with the Boeing 757 & 767 as you only have an STC, Boeing does not fit them on the 757 & 767 only on the 737NG.
The Airbus fit is part of the TC, so l can not see a STC being developed in the near future.
Airbus is the same, major wing strengthening as per the APB modifications, it is amazing the area of the a/c that is covered by reduced MRBR/MPD intervals, including a lot of the fuse.
Increased maintenance cost which is never mentioned, but reduced fuel burn as you have rightly pointed out.