GJB,
Agree totally with your point. The fact is that winglets are an experimental development by the manufacturers. Thus, the likelyhood of obtaining any conclusive information on their performance from them is slim!!
I remember reading an article some months ago (in Airliner World or something) that showed a BA 744 operating with one winglet! From that, I imagine that the effect of the winglet could not be very significant!
Cheers,
Cuban_8