DxOMark has just released the test scores of RX1 sensor.

RX1 sensor is rated #4 for Overall score (93) of all camera sensors they have ever tested, including medium format sensors.

RX1 sensor is #3 in Dynamic range of 14.3 EVs
RX1 sensor is #7 in low light score of 2534 ISO
RX1 sensor is #7 in Color Depth of 25.1 bits

This shot was taken hand-held and the tree trunk was lit by a street lamp. At ISO 6400, there was some noise (luminance noise only) but the resolution was not comprised.

The factory default maximum Auto ISO for RX1 is set at 6400. With enough light for a "proper" exposure, the camera always use a shutter speed of 1/80 second. When the light level is so dim that the maximum ISO is reached, the camera will then choose a shutter speed slower than 1/80 second.
In this image, the aperture was wide open at f/2.0, the set maximum Auto ISO of 6400 was reached, the camera chose a shutter speed of 1/13s, one can visualize how dim the scene was.