To the Bio-warfare community “Gain of Function” (which is the proper term) is a desired characteristic for the weaponization of a pathogen.
Im pretty sure bioweapon people would start with a fairly accomplished killer from the outset. Which is why real bioweapon are Anthrax, especially the spores and Bot Tox.
If you read up on gain of function they use weak pathogens on purpose. And gain of function isnt new technology gene manipulation but basically torturing the virus which is the natural genomic equivalent of an Opossum into behaving like a Lion. Which is why people can honestly say there is no evidence of bioengineering. Because its true.
Heres an article on How to do it, pretty technical but its all right there.
https://www.sciencedirect.com/science/article/pii/S0065352706660053