DxOMark published their Leica S camera test results, here is their conclusion:
In the rarified atmosphere of professional medium format cameras, the Leica S performs very well indeed. It has both very good color and dynamic range and, surprisingly perhaps, it has excellent low noise characteristics. While it’s clear smaller 35mm full-frame CMOS sensors have better dynamic range and lower noise at ultra-high ISOs, the Leica S is delivers on its promise as being just as adept in the field as it is in the studio.