My knee-jerk reaction is that having these standards may indeed reduce overall casualties

I think that's the argument that by default requires proof, not the other way around.

I would say the opposite, out of conservatism, but I don't expect to be able to argue the point as anything more than headbutting intuitions.

If my goal is to kill while also killing few civilians, I'll go for that, but I'll do it more efficiently in most cases than if I have to follow laws I don't believe in.

True of normative reasoners, not of humans. See Ethical Inhibitions.