Would it be possible to have the same behaviour in Sonar Viewer as in Reefmaster, where the movement of the map is synced to what is shown in the side- and downscan plots? Now, one has to manually move the centre of the map to see what's displayed in the Sidescan plot, when zoomed in the map.

.. or have I missed a setting somewhere?

All of the new Sonar Viewer features will eventually make it into ReefMaster. Although you can't currently sync sonar playback to screen position in RM, you can use the "swath control" at the bottom of the map view to lock onto the front of the swath - just click the button at the left of this bar and then adjust the swath position by using the slider.