Any aircraft designed to a span limitation can have its max L/D increased by adding winglets; if done intelligently the L/D increase more than pays for the additional wing weight due to the higher bending moments. This was true for the A380, which was designed to an 80m span limit when the optimum span for the aircraft was somewhere around 84m.
Even on an aircraft which is not span limited, winglets can be used with minor wing weight penalty by soaking up some of the 'fat' in the wing strength which is evident once the ultimate static load tests are done; any strength margin over the certification requriements can be cashed in as an aero benefit via winglets. On the A340-300 the winglets were added mainly to improve the low speed performance, but they also cut cruise drag by about 3-4%. Aviation Partners claim around 7% for the BBJ winglets, but they are proportionately twice the height of the A340 winglets.
Some winglet effects are not immediately obvious. One project I worked on gained significant wing limit load relief due to the wing sweep and fact that the aft-positioned winglet pulled the aero centre aftwards at the wingtip which caused the load to shift further inboard at manoeuvre limit due to aeroelastic wing twist. The weight saving more that balanced the lower aero benefit of this smaller winglet. All jolly interesting.
On Glide