It's all been worthwhile, but no retro-fitting seems to be in order.
No first hand knowledge here (after all, I work for the competition), but on most Boeing aircraft the retrofit of winglets drives internal structural changes to the wing because they change the load distribution across the wing. I'm guessing that sharklets have a similar effect.
A non-Boeing buddy of mine worked the STC for the 757 and 767 winglets and he said the internal wing changes are substantial. On the plus side, fuel burn improvement for 3+ hour flights will pay for it in a year, even though the retrofit cost is seven figures.
If they are really getting 4% for the sharklets, someone will do the work to retrofit them via STC.