I think that when you look at the bottom line, you have to consider exactly when the advertised benefits occur. The 5%-9% isnt for all phases of flight.
From Boeings chart, you can see that on take-off, there is little benefit.
From what it looks like to me, the benefit of winglets comes from a reduction in the angle of attack.
So for en-route, at the beginning when heavy, a decreased angle here would appear to be the most benefit. As the fuel load lightens, the angle would normally decrease, but there is a limit, or minimum angle for both with/without winglets, so, the benefits equal out at some point.
I suppose someone could actually calculate this out, and provide a real cost benefit analysis. So if the airline is really serious about the expenditure, perhaps this is the way to approach the issue.
In regards to the winglets providing a dampening effect in turbulence, I have not heard anyone say very much about this.
Swept wingtips and x wing configurations cloud the issue as well....