I have added the felling quality to all plasma weapons as it makes them even more effective against high value targets and makes a lot of sense when you think about a ball of condensed hydrogen molecules that are superheated to over 6000 degrees Celsius hitting a man’s flesh. I may add this property to melta weapons as well but I haven't done the math for it. The significantly reduced range and limited rate of fire seem like significant enough penalties in and of themselves anyway.
I have also house ruled the felling property eliminates toughness when there is no unnatural trait.