We had one BA's 744's arrive one morning (when they were still on the sttrength at LGW!) minus 1 winglet and the crew explained that the fuel burn penalty was less than 1%. No discernable asymetric effects and they confirmed they departed knowing it was absent.
I guess that they make a difference to fuel burn over the lifetime of the a/c but not enough to affect an individual flight.
Certianly a test for our powers of observation at the end of a night shift, though!!
The Odd One