What I'm getting at is: the only real cost of softening one's tone is a slight reduction in efficiency.

I've discussed the costs elsewhere. I'd add the game theoretic costs of getting into a "I'm offended, you have to change" game. The costs of annoying and/or offending someone who doesn't appreciate being emotionally handled. The cost of not conveying your actual personality in the conversation. The non trivial cost of always maintaining two channels in every conversation - topic and niceness. Even if "only a little" niceness is required, the mental attention required likely has some floor.

one also has to put some effort into determining where one sits on the overall scale of politeness-expectation, and accommodating those who are higher up the scale

I think that's reasonable. In the case of a shared space, some tradeoff and consideration is expected on all sides.