Expected better handling. Expected better rear end geometry.
Also gives me something extra to talk about

With a panhard rod on a 1200 sedan, it is difficult to place the bar in a way that will ensure that the diff goes generally up and down. The watts linkage appears to be an easier (not easy) way of achieving this.
I know that my current setup makes the housing move through an arc, so that when it's at the bottom of it's travel, it rubs on one sides inner guard. That's why we put the adjustable bit in the centre of the panhard rod to 'readjust' the central position depending on the ride height.
The racing guys place the centre pivot onto an adjustable mount on the body, and the arms onto the diff. This allows adjustment of the roll centre from in the cabin, as they have levers/actuators to move the centre link up and down depending on the track conditions/corner.
As the 1200 isn't as top heavy as a falcon, so the position of the roll centre for the falcon diff should be nice and low for the 1200.
So that's my current thinking in regards to the watts linkage.