I think most of the work of rendering web content is in layout and font rendering, plus JaveScript, etc – none of which benefits from OpenGL. The page you link suggests they draw the finished render to screen using OpenGL, but I don’t know if they use OpenGL to actually do the rendering.
Another option is CoGeWebKit which should be more efficient than any of the screen-grabbing methods.
VDMX also has an web renderer which works well – presumably also WebKit-based.
