Have you looked at the likely impacts [www.ipcc.ch] with a mere 1K of warming?
I can look at first hand experience because we have had 1C of warming over the last 150 years. By pretty much all objective measure life is better for the vast majority of the world's population.
in fact, competent disaster planning would more likely work on the assumption of the worse case
Mitigation is not free. It actually has a very high cost all of it's own. An actuarial cost-benefit analysis is not part of science and should be done by economists, not scientists.
the only thing that's completely "settled" is whether it's happening at all
It is warming. It has been warming since the mid 1800s, and indeed for the last 12,000 years given it is an interglacial. Even if there was no science at all my money would be bet on more warming.
though we've got a pretty good idea about how & why
The lack of predictive ability belies that.