How much lift do supercritical airfoils produce?
Supercritical airfoils produce comparably more lift than conventional cambered airfoils but less than optimised high-lift airfoils. This is because


they have blunter noses which work over a wider angle of attack range,
the blunt nose also allows to use effective slats or Krüger flaps,
they have plenty of camber in the aft section of the airfoil and
the high rear loading allows for thin, highly cambered flaps.


From history.nasa.gov: 


  In the course of developing the wings for these flight programs, it was learned that the supercritical airfoils had excellent high lift characteristics because of their large leading-edge radii. This important benefit tended to offset the fact that their subcritical profile drag is higher than for comparable 6-series sections.


However, the forward part contributes relatively little to lift because it has negative camber. Also, the rear loading causes a high pitching moment.

In the supercritical speed region their upper side produces more suction since local speeds can reach up to Mach 1.3 while conventional airfoils must struggle to keep speeds subsonic. Part of that advantage is used up by making the airfoil thicker so suction on the lower side is also higher than on conventional airfoils, but an advantage remains. More recent airfoils add some more lift at the nose by designing a slow speed rise on the lower side while the upper side accelerates the local flow more quickly. Their main lift, however, comes from the thin, highly cambered aft section.

When R. Whitcomb researched supercritical airfoils, he noticed their high subsonic lift and modified the general shape by adding more forward camber into the GA(W)-1 and -2 (General Aviation Whitcomb) airfoils which became popular choices for modern GA aircraft due to their high lift and benign stall characteristics (witness 93 hits on Dave Lednicer's incomplete guide to airfoil usage, among them the Piper PA-38, the Glasair and the Edgley Optica).