Thank you. I still want to try debayering the corners of the sensor but just don't feel brave enough to do it
For 2x2 binning the sensor adds the 4 adjacent pixels together into a super-pixel. This means that the RGGB pixels defined by the Bayer matrix get added together and hence the colour information gets lost in the process. Basically I got 1/4 intensity and 1/4 resolution
At least the read noise was 1/4 compared to unbinned image