The internal clock to bitbang the sensor is forced to 500 kHz maximum, recommended frequency by Mitsubishi. It is possible to downclock the sensor (this is the principle of the "nigth mode" feature), but not overclocking it (this produces images with dark halo in the upper side as the photon collection overlaps with the pixel reading). Of course this is written nowhere in the datasheet.

The image writing to SD is the bottleneck of the device, it cannot be improved by overclocking, at least with my current knowledge. This locks the writing speed to more or less to 4 fps. It would be possible to compress data with the Game Boy Tile Format to increase writing speed but this would be possible only with dithering mode on, so a complicated feature to implement for very little added value.