I agree with tarnished there is no straight forward answer. For any wing cl/cd changes with IAS and Alpha. So if you want to define efficient you will have to specify an altitude a speed and a weight and then compare the various flap settings.
There is a mod around for 73s or 72s that deploys the flaps by a couple of degrees in the cruise to reduce drag. I think it floats the ailerons up a bit also. This shows that at the average cruise conditions the aircraft is used the wing as originally designed is not at optimal Cl/Cd but by making use of the flaps it can be improved.
Think the article was in Flight International a month or so ago.