I created a request for this ages ago, but can't find it now.
In short, the idea is:
- create a simple animation of flying red chunks of flesh
- make this animation play when overkill happens
- recolour the animation if needed (for example to green for Mutons, to black for machines, etc.; would need setting it individually on armours or maybe units)
A more ambitious version:
- as above, but introduce multiple overkill animations (melts, turns to ash, etc.) - set by weapon clip