Thus, I can never be more than one minus one-in-ten-billion sure that my sensory experience is even roughly correlated with reality. Thus, it would require extraordinary circumstances for me to have any reason to worry about any probability of less than one-in-ten-billion magnitude.

No. The reason not to spend much time thinking about the I-am-undetectably-insane scenario is not, in general, that it's extraordinarily unlikely. The reason is that you can't make good predictions about what would be good choices for you in worlds where you're insane and totally unable to tell.

This holds even if the probability for the scenario goes up.