The current winglets on the 747-400 are more for show than anything else. Call them the golden rivet that allowed for changing the way that Boeing certified performance for the bird. The BBJ actually only gets about 4.5% to 5.5% depending on CG and other conditions. Low speed aero performance is enhanced on most aircraft so MTOW can be greater out of runway limited airports. They also have some noise benefits. The new raked ones are the "tweak of the week" at Boeing. They are no more efficient than a winglet, in fact they do the same thing: increase wing span loading further out from the plane. In rear engined planes at heavy aft CG they can cause problems in stall recovery. If they could design them to last and not fall off the most effiecient wing design known to man would look like the outer part of a turkey wing when inflight. You will find that trim drag has a far greater affect on cruise performance than those winglets.