I can't dispute what Zleica said about the Summarit but my experience with the Sonnar was it was exceptionally sharp at the center, even wide open, with softening at the edge of the frame, much like a good Tessar copy such as the 45/2.8 Nikkor AI-P. Peak sharpness over most of the frame was by f/4 or f/5.6. One reason it was so sharp was that DAG tweaked to focus to match Leica tolerances. Possibly some copies out their might require a similar adjustment. The Summarit 40/2.4 benefits slightly from multi-coating over the 40/2 Summicon giving images a little more punch.
Also, how do you adjust the aperture on the 40/2.4?