I don't have the numbers handy, but for the MD-11 the lack of even a lower winglet adds a fair amount. Sure, you can operate that way, but you must take into account the increased fuel burn. The numbers are well known and contained in the configuration deviation list (CDL) which is FAA approved in the U.S. by the FSB and FOEB process. The MMEL is also created in this way, and these docs are adopted by most operators around the world.