What is the average weight difference between composite and metal airliners?
(Own work via boeing.com) Overlayed scaled drawings.

Above you see similar sized planes with the same 3-class seating (± 1 seat). The newer-more-composite 787 is 16 tonnes heavier, yet delivers up to 20% fuel saving (route dependent). 3.4 of the 16 tonnes are the newer and bigger engines. And the rest is primarily bigger wings (which can be built bigger more easily thanks to composites).

The aim is building bigger wings, not losing weight. The other main goal is the reduced maintenance cost (better fatigue tolerance, no rust, etc.)

Boeing says:


  The result is an airframe comprising nearly half carbon fiber reinforced plastic and other composites. This approach offers weight savings on average of 20 percent compared to more conventional aluminum designs.


Based on that quotation, the answer is -20% (given the same wing, etc.). Also note the 787 also saves weight from the no-bleed architecture, yet as the above shows, it's not a lighter plane than the one it replaces. (No-bleed also improves engine fuel efficiency, not just weight.)