The reality with Leica lenses, as with any other lens, is that it will not, cannot, be as sharp fully wide open as stopped down a bit. That said Leica lenses tend to be more sharp than others wide open. So you have a balancing act. To say it is sharpest wide open is a fantasy, even Leica never aspired to that, f/4 being the datum point if your preffered DOF coincides with that, but any other aperture is perfectly acceptable without any big traumas.